Q:

NullPointerException when I show a stage in JavaFX

I'm using JavaFX and I have an exception when I'm trying to show a stage.
How can I know what is the problem? The line is in: this.stage.show();
I got that in the console:
Exception in Application start method
Exception in thread "JavaFX Application Thread" java.lang.NullPointerException
    at com.sun.javafx.scene.control.skin.ChoiceBoxSkin.updatePopupItems(Unknown Source)
    at com.sun.javafx.scene.control.skin.ChoiceBoxSkin.initialize(Unknown Source)
    at com.sun.javafx.scene.control.skin.ChoiceBoxSkin.<init>(Unknown Source)
    at javafx.scene.control.ChoiceBox.createDefaultSkin(Unknown Source)
    at javafx.scene.control.Control.impl_processCSS(Unknown Source)
    at javafx.scene.Parent.impl_processCSS(Unknown Source)
    at javafx.scene.Parent.impl_processCSS(Unknown Source)
    at javafx.scene.Parent.impl_processCSS(Unknown Source)
    at javafx.scene.Node.processCSS(Unknown Source)
    at javafx.scene.Node.processCSS(Unknown Source)
    at javafx.scene.Node.processCSS(Unknown Source)
    at javafx.scene.Node.processCSS(Unknown Source)
    at javafx.scene.Node.processCSS(Unknown Source)
    at javafx.scene.Scene.doCSSPass(Unknown Source)
    at javafx.scene.Scene.access$3600(Unknown Source)
    at javafx.scene.Scene$ScenePulseListener.pulse(Unknown Source)
    at com.sun.javafx.tk.Toolkit.lambda$runPulse$31(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at com.sun.javafx.tk.Toolkit.runPulse(Unknown Source)
    at com.sun.javafx.tk.Toolkit.firePulse(Unknown Source)
    at com.sun.javafx.tk.quantum.QuantumToolkit.pulse(Unknown Source)
    at com.sun.javafx.tk.quantum.QuantumToolkit.pulse(Unknown Source)
    at com.sun.javafx.tk.quantum.QuantumToolkit.lambda$runToolkit$405(Unknown Source)
    at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(Unknown Source)
    at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at com.sun.glass.ui.win.WinApplication.lambda$null$149(Unknown Source)
    at java.lang.Thread.run(Unknown Source)
java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at com.sun.javafx.application.LauncherImpl.launchApplicationWithArgs(Unknown Source)
    at com.sun.javafx.application.LauncherImpl.launchApplication(Unknown Source)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at sun.launcher.LauncherHelper$FXHelper.main(Unknown Source)
Caused by: java.lang.RuntimeException: Exception in Application start method
    at com.sun.javafx.application.LauncherImpl.launchApplication1(Unknown Source)
    at com.sun.javafx.application.LauncherImpl.lambda$launchApplication$156(Unknown Source)
    at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.NullPointerException
    at com.sun.javafx.scene.control.skin.ChoiceBoxSkin.updatePopupItems(Unknown Source)
    at com.sun.javafx.scene.control.skin.ChoiceBoxSkin.initialize(Unknown Source)
    at com.sun.javafx.scene.control.skin.ChoiceBoxSkin.<init>(Unknown Source)
    at javafx.scene.control.ChoiceBox.createDefaultSkin(Unknown Source)
    at javafx.scene.control.Control.impl_processCSS(Unknown Source)
    at javafx.scene.Parent.impl_processCSS(Unknown Source)
    at javafx.scene.Parent.impl_processCSS(Unknown Source)
    at javafx.scene.Parent.impl_processCSS(Unknown Source)
    at javafx.scene.Parent.impl_processCSS(Unknown Source)
    at javafx.scene.Parent.impl_processCSS(Unknown Source)
    at javafx.scene.Parent.impl_processCSS(Unknown Source)
    at javafx.scene.Parent.impl_processCSS(Unknown Source)
    at javafx.scene.Node.processCSS(Unknown Source)
    at javafx.scene.Scene.doCSSPass(Unknown Source)
    at javafx.scene.Scene.preferredSize(Unknown Source)
    at javafx.scene.Scene.impl_preferredSize(Unknown Source)
    at javafx.stage.Window$9.invalidated(Unknown Source)
    at javafx.beans.property.BooleanPropertyBase.markInvalid(Unknown Source)
    at javafx.beans.property.BooleanPropertyBase.set(Unknown Source)
    at javafx.stage.Window.setShowing(Unknown Source)
    at javafx.stage.Window.show(Unknown Source)
    at javafx.stage.Stage.show(Unknown Source)
    at view.implementations.View.loadConfiguration(View.java:76)
    at main.Main.start(Main.java:40)
    at com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$163(Unknown Source)
    at com.sun.javafx.application.PlatformImpl.lambda$runAndWait$176(Unknown Source)
    at com.sun.javafx.application.PlatformImpl.lambda$null$174(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at com.sun.javafx.application.PlatformImpl.lambda$runLater$175(Unknown Source)
    at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(Unknown Source)
    at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at com.sun.glass.ui.win.WinApplication.lambda$null$149(Unknown Source)
    ... 1 more
Exception running application main.Main

A little help? I see that the problem is in one of the ChoiceBox. They are an observableList.
Elements:
@FXML
private ChoiceBox<String> choiceBox1;
private ObservableList<String> observableList1;

Where I initialize:
List<String> list1 = new ArrayList<String>();
    list1.add("1");
    list1.add("2");
    list1.add("3");
    this.observableListHash = FXCollections.observableList(list1);
this.choiceBox1.setItems(this.observableList1);
this.choiceBox1.setValue("1");

I set a value because I want a predeterminated value.

A:

You don't initalize observableList1 and you add elements to another list.
This ...
List<String> list1 = new ArrayList<String>();
list1.add("1");
list1.add("2");
list1.add("3");
this.observableListHash = FXCollections.observableList(list1);
this.choiceBox1.setItems(this.observableList1);
this.choiceBox1.setValue("1");

... should be ...
observableList1 = FXCollections.observableArrayList();
observableList1.addAll("1", "2", "3");
this.choiceBox1.setItems(observableList1);
this.choiceBox1.setValue("1");

