Q:

Importing int variable from another class crashes app

I'm trying to design a times table testing app, the theory is that the user is presented with a screen with 12 buttons to select from. 
When he presses a button it opens a new class testing him with the number he chose.  There are 4 main files:

MainActivity.java
activity_main.xml
Times.java
activity_times.java

Currently the class view that comes up to test the user (activity_times.xml), just contains three text views:

"HI" id=textView
"" (empty to be edited in the java class) id=TextOut
"Under development...." id=textView2

The first and last text views are not important, they'll be removed later on.
In the MainActivity.java class I declared a shared variable
public int numberClicked;

and then told the buttons to set the variable to a certain number:
        button2 = findViewById(R.id.button2);
        button2.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                numberClicked = 2;
                openTestPage();

            }
        });

and then on the Times.java class I told it to import it and the empty output text view:
private MainActivity MA;
private int SubjectNumber = MA.numberClicked;
private final TextView OutputText = findViewById(R.id.TextOut);

and then to set the text view to the imported variable:
OutputText.setText("You chose the number " + SubjectNumber);

The output I want is that on the screen is that the TextOut text view shows, for example if the user presses the button with the number "2" on it: "You chose the number 2".

But instead of that, the app crashes when the second class view launches :(
 To stop it from crashing I have to comment out the following code:
private MainActivity MA;
private int SubjectNumber = MA.numberClicked;
private final TextView OutputText = findViewById(R.id.TextOut);

If I uncomment only one of the three lines it will still crash.
I looked in the logcat and saw something interesting:
2020-08-23 15:20:18.553 6537-6537/? E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.ma, PID: 6537
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.ma/com.example.ma.Times}: java.lang.NullPointerException: Attempt to read from field 'int com.example.ma.MainActivity.numberClicked' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2844)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3049)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1809)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:193)
        at android.app.ActivityThread.main(ActivityThread.java:6680)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
     Caused by: java.lang.NullPointerException: Attempt to read from field 'int com.example.ma.MainActivity.numberClicked' on a null object reference
        at com.example.ma.Times.<init>**(Times.java:13)**
        at java.lang.Class.newInstance(Native Method)
        at android.app.AppComponentFactory.instantiateActivity(AppComponentFactory.java:69)
        at androidx.core.app.CoreComponentFactory.instantiateActivity(CoreComponentFactory.java:41)
        at android.app.Instrumentation.newActivity(Instrumentation.java:1215)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2832)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3049) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1809) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:193) 
        at android.app.ActivityThread.main(ActivityThread.java:6680) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858) 

Every thing that came out of the logcat you will find here.
My AndroidManefest.xml:
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.ma">

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="Times Practice"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name=".Times">
        </activity>
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>

        <meta-data
            android:name="preloaded_fonts"
            android:resource="@array/preloaded_fonts" />
    </application>

</manifest>

My MainActivity.java
package com.example.ma;

import androidx.appcompat.app.AppCompatActivity;

import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;

public class MainActivity extends AppCompatActivity {
    public int numberClicked;
    private Button button2;
    private Button button3;
    private Button button4;
    private Button button5;
    private Button button6;
    private Button button7;
    private Button button8;
    private Button button9;
    private Button button10;
    private Button button11;
    private Button button12;
    private Button button13;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        button2 = findViewById(R.id.button2);
        button2.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                numberClicked = 2;
                openTestPage();

            }
        });

        button3 = findViewById(R.id.button3);
        button3.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                numberClicked = 3;
                openTestPage();
            }
        });

        button4 = findViewById(R.id.button4);
        button4.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                numberClicked = 4;
                openTestPage();
            }
        });

        button5 = findViewById(R.id.button5);
        button5.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                numberClicked = 5;
                openTestPage();
            }
        });

        button6 = findViewById(R.id.button6);
        button6.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                numberClicked = 6;
                openTestPage();
            }
        });

        button7 = findViewById(R.id.button7);
        button7.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                numberClicked = 7;
                openTestPage();
            }
        });

        button8 = findViewById(R.id.button8);
        button8.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                numberClicked = 8;
                openTestPage();
            }
        });

        button9 = findViewById(R.id.button9);
        button9.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                numberClicked = 9;
                openTestPage();
            }
        });

        button10 = findViewById(R.id.button10);
        button10.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                numberClicked = 10;
                openTestPage();
            }
        });

        button11 = findViewById(R.id.button11);
        button11.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                numberClicked = 11;
                openTestPage();
            }
        });

        button12 = findViewById(R.id.button12);
        button12.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                numberClicked = 12;
                openTestPage();
            }
        });

        button13 = findViewById(R.id.button13);
        button13.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                numberClicked = 13;
                openTestPage();
            }
        });

    }
    public void openTestPage() {
        Intent intent = new Intent(this, Times.class);
        startActivity(intent);
    }
}

My activity_main.xml:
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <TextView
        android:id="@+id/Instructions"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:fontFamily="@font/acme"
        android:gravity="center"
        android:text="Choose the number you want to practice your times tables with....."
        android:textAppearance="@style/TextAppearance.AppCompat.Large"
        android:textSize="36sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.0"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.051" />

    <Button
        android:id="@+id/button2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:fontFamily="@font/architects_daughter"
        android:text="2"
        android:textSize="20sp"
        android:textStyle="bold"
        app:layout_constraintBottom_toTopOf="@+id/button5"
        app:layout_constraintEnd_toStartOf="@+id/button3"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/Instructions" />

    <Button
        android:id="@+id/button3"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:fontFamily="@font/architects_daughter"
        android:text="3"
        android:textSize="20sp"
        android:textStyle="bold"
        app:layout_constraintBottom_toTopOf="@+id/button6"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.501"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/Instructions" />

    <Button
        android:id="@+id/button4"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:fontFamily="@font/architects_daughter"
        android:text="4"
        android:textSize="20sp"
        android:textStyle="bold"
        app:layout_constraintBottom_toTopOf="@+id/button7"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.493"
        app:layout_constraintStart_toEndOf="@+id/button3"
        app:layout_constraintTop_toBottomOf="@+id/Instructions" />

    <Button
        android:id="@+id/button5"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:fontFamily="@font/architects_daughter"
        android:text="5"
        android:textSize="20sp"
        android:textStyle="bold"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toStartOf="@+id/button6"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.499" />

    <Button
        android:id="@+id/button6"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:fontFamily="@font/architects_daughter"
        android:text="6"
        android:textSize="20sp"
        android:textStyle="bold"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.501"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.499" />

    <Button
        android:id="@+id/button7"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:fontFamily="@font/architects_daughter"
        android:text="7"
        android:textSize="20sp"
        android:textStyle="bold"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.493"
        app:layout_constraintStart_toEndOf="@+id/button6"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/button8"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:fontFamily="@font/architects_daughter"
        android:text="8"
        android:textSize="20sp"
        android:textStyle="bold"
        app:layout_constraintBottom_toTopOf="@+id/button11"
        app:layout_constraintEnd_toStartOf="@+id/button9"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/button5" />

    <Button
        android:id="@+id/button9"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:fontFamily="@font/architects_daughter"
        android:text="9"
        android:textSize="20sp"
        android:textStyle="bold"
        app:layout_constraintBottom_toTopOf="@+id/button12"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/button6" />

    <Button
        android:id="@+id/button10"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:fontFamily="@font/architects_daughter"
        android:text="10"
        android:textSize="20sp"
        android:textStyle="bold"
        app:layout_constraintBottom_toTopOf="@+id/button13"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toEndOf="@+id/button9"
        app:layout_constraintTop_toBottomOf="@+id/button7" />

    <Button
        android:id="@+id/button11"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="80dp"
        android:fontFamily="@font/architects_daughter"
        android:text="11"
        android:textSize="20sp"
        android:textStyle="bold"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toStartOf="@+id/button12"
        app:layout_constraintStart_toStartOf="parent" />

    <Button
        android:id="@+id/button12"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="80dp"
        android:fontFamily="@font/architects_daughter"
        android:text="12"
        android:textSize="20sp"
        android:textStyle="bold"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent" />

    <Button
        android:id="@+id/button13"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="80dp"
        android:fontFamily="@font/architects_daughter"
        android:text="13"
        android:textSize="20sp"
        android:textStyle="bold"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toEndOf="@+id/button12" />

</androidx.constraintlayout.widget.ConstraintLayout>

My Times.java:
package com.example.ma;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.widget.TextView;

public class Times extends AppCompatActivity {

    private MainActivity MA;
    private int SubjectNumber = MA.numberClicked;
    private final TextView OutputText = findViewById(R.id.TextOut);

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_times);
        OutputText.setText("You chose the number " + SubjectNumber);

    }
}

My activity_times.xml:
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".Times">

    <TextView
        android:id="@+id/textView2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Under development...."
        android:textSize="28sp"
        android:textStyle="bold"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.503"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.696" />

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="HI"
        android:textSize="100sp"
        app:layout_constraintBottom_toTopOf="@+id/TextOut"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.501"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <TextView
        android:id="@+id/TextOut"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="316dp"
        android:textSize="30sp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>

A:

You shouldn't try to directly access an Activity from another Activity. In Android, you add the data you want to share with another Activity onto an Intent. You also shouldn't try to obtain a view outside of the Activity lifecycle. Your findViewById() should be moved to the Activity's onCreate().
In your first Activity's openTestPage():
Intent intent = new Intent(this, Times.class);

// Add the value you want to pass to the other Activity
// to the Intent
intent.putExtra("NUMBER_CLICKED", numberClicked);

startActivity(intent);

Your second Activity should look something like this:
public class Times extends AppCompatActivity {

    private int SubjectNumber;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_times);
        
        // Access your views here
        TextView OutputText = findViewById(R.id.TextOut);
        
        // Retrieve the value passed into the Intent
        // (The 0 is a default value to fallback on. You can use another value.)
        SubjectNumber = getIntent().getIntExtra("NUMBER_CLICKED", 0);
        
        OutputText.setText("You chose the number " + SubjectNumber);
    }
}

