Q:

Receiving an error in Java and don't understand the problem: Exception in Application init method java.lang.reflect.InvocationTargetException

I'm developing a program using JavaFX and I see no errors in my code (not underlined), yet when I run the code using a seperate file, I am getting the following error:
Here is the full stack trace:
Exception in Application init method
java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at com.sun.javafx.application.LauncherImpl.launchApplicationWithArgs(LauncherImpl.java:389)
    at com.sun.javafx.application.LauncherImpl.launchApplication(LauncherImpl.java:328)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at sun.launcher.LauncherHelper$FXHelper.main(Unknown Source)
Caused by: java.lang.RuntimeException: Exception in Application init method
    at com.sun.javafx.application.LauncherImpl.launchApplication1(LauncherImpl.java:912)
    at com.sun.javafx.application.LauncherImpl.lambda$launchApplication$1(LauncherImpl.java:182)
    at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.IllegalArgumentException: Children: duplicate children added: parent = Grid hgap=0.0, vgap=0.0, alignment=TOP_LEFT
    at javafx.scene.Parent$2.onProposedChange(Parent.java:454)
    at com.sun.javafx.collections.VetoableListDecorator.add(VetoableListDecorator.java:206)
    at website.CreateAccountPane.<init>(CreateAccountPane.java:60)
    at website.WebsiteRootPane.<init>(WebsiteRootPane.java:22)
    at main.ApplicationLoader.init(ApplicationLoader.java:14)
    at com.sun.javafx.application.LauncherImpl.launchApplication1(LauncherImpl.java:841)
    ... 2 more
Exception running application main.ApplicationLoader

This is the code for my application launcher:
package main;

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.stage.Stage;
import website.WebsiteRootPane;

public class ApplicationLoader extends Application {

    private WebsiteRootPane root;

    @Override
    public void init() {
        root = new WebsiteRootPane();
    }

    @Override
    public void start(Stage stage) throws Exception {
        stage.setMinWidth(1000);
        stage.setMinHeight(750);
        stage.setTitle("Planet Generator");
        stage.setScene(new Scene(root));
        stage.show();
    }

    public static void main(String[] args) {
        launch(args);
    }

}

This is one of the tab panes for my website, so far it is the only one I've built.

package website;

import javafx.collections.FXCollections;
import javafx.collections.ObservableList;

import javafx.geometry.Insets;

import javafx.geometry.Pos;

import javafx.scene.control.Button;

import javafx.scene.control.ComboBox;

import javafx.scene.control.Label;

import javafx.scene.control.PasswordField;

import javafx.scene.control.TextField;

import javafx.scene.layout.GridPane;

public class CreateAccountPane extends GridPane {

    private ComboBox<String> cboTitle;
    private TextField txtSurname, txtFirstName, txtEmail;
    private Button btnCreate;
    private PasswordField pwrd;

    public CreateAccountPane() {

        //create labels
        Label lblTitle = new Label("Title");
        Label lblFirstName = new Label("First name");
        Label lblSurname = new Label("Surname");
        Label lblEmail = new Label("Email");
        Label lblPassword = new Label("Password");

        // setup combobox
        ObservableList<String> titles = FXCollections.observableArrayList("Mr", "Mrs", "Miss", "Ms");
        cboTitle = new ComboBox<String>(titles);
        cboTitle.getSelectionModel().select(0);

        // setup text fields
        txtFirstName = new TextField();
        txtSurname = new TextField();
        txtEmail = new TextField();
        pwrd = new PasswordField();

        this.add(lblTitle, 0, 0);
        this.add(cboTitle, 1, 0);

        this.add(lblFirstName, 0, 1);

        this.add(txtFirstName, 1, 1);

        this.add(lblSurname, 0, 2);
        this.add(txtSurname, 1, 2);

        this.add(lblEmail, 0, 3);
        this.add(txtEmail, 1, 3);

        this.add(lblPassword, 0, 4);
        this.add(pwrd, 1, 4);

        this.getChildren().add(txtFirstName);       
    }
}

A:

The problem
You are adding txtFirstName twice but you can only add an element to the UI once.
You add it with
this.add(txtFirstName, 1, 1);

and
this.getChildren().add(txtFirstName);

You should be able to fix it by removing
this.getChildren().add(txtFirstName);

How to find that (read that, it will help you :))
Look at your exception:

Exception in Application init method java.lang.reflect.InvocationTargetException

This says that the problem occurs while executing the code of init.
InvokedTargetException means that a method invoked using reflection(what is the case with init as JavaFX uses reflection for this) threw an exception. This could be any exception. The Caused by section in the stack trace shows the real exception. Let's look at it:

Caused by: java.lang.IllegalArgumentException: Children: duplicate children added: parent = Grid hgap=0.0, vgap=0.0, alignment=TOP_LEFT 
  at javafx.scene.Parent$2.onProposedChange(Parent.java:454) 
  at com.sun.javafx.collections.VetoableListDecorator.add(VetoableListDecorator.java:206)
  at website.CreateAccountPane.(CreateAccountPane.java:60)

The exception message (Children: duplicate children added: parent = Grid hgap=0.0, vgap=0.0, alignment=TOP_LEFT) tells you that the problem is that you have already added that element.

at website.CreateAccountPane.(CreateAccountPane.java:60)

tells you that it occured in line 60 of CreateAccountPane(CreateAccountPane.java:60).
Next, we look closer at this line:
this.getChildren().add(txtFirstName);

It actually adds an element to the grid pane.
So, it seems that this element is already a part of the grid pane. So, let's see what you've added to it previously:
this.add(txtFirstName, 1, 1);

You added the same element two times. That is the problem.

Conclusion
As you see, reading and interpreting stack traces is a very, very important part of programming. If you go on, you will see more and more stack traces like this.
Don't fear those huge red texts. Split them apart and try to find the problem.
This may be time consuming and difficult, especially at the beginning but with time, you will get better!
Finding, understanding and fixing bugs is a very important part of programming(maybe the most important part).
This part will be much easier if you can 

read and interpret exceptions

and

debug. If you don't know what debugging is, look it up, learn it and try it out. It will almost surely help you.

