Run code:


import javafx.application.Application;
import javafx.beans.value.ChangeListener;
import javafx.beans.value.ObservableValue;
import javafx.geometry.Orientation;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.ToggleButton;
import javafx.scene.control.ToolBar;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

public class NewClass extends Application {

    public static void main(String[] args) {
        launch(args);
    }
    ToolBar bar = new ToolBar();
    ToggleButton shrinkButon;

    @Override
    public void start(Stage stage) throws Exception {
        VBox vb = new VBox();

        bar = new ToolBar();
        bar.setOrientation(Orientation.VERTICAL);

        shrinkButon = new ToggleButton();
        shrinkButon.selectedProperty().addListener(new ChangeListener() {

            @Override
            public void changed(ObservableValue ov, Object t, Object t1) {
                setSizes();
            }
        });

        vb.getChildren().addAll(bar, shrinkButon);

        reset();

        Scene scene = new Scene(vb, 300, 500);
        stage.setScene(scene);
        stage.show();
    }

    protected void reset() {
        shrinkButon.setSelected(false);
        bar.getItems().clear();
        for (int i = 0; i < 10; i++) {
            Button btn;
            setSizes();
            bar.getItems().add(btn = new Button("Button " + i) {

                @Override
                public void fire() {
                    System.out.println(getId());
                }
            });
            btn.setId(String.valueOf(i));
        }
    }

    protected void setSizes() {
        if (shrinkButon.isSelected()) {
            bar.setPrefSize(150, 80);
            bar.setMaxSize(150, 80);
            bar.setMinSize(150, 80);
        } else {
            bar.setPrefSize(150, 400);
            bar.setMaxSize(150, 400);
            bar.setMinSize(150, 400);
        }
    }
}

Click toggle button to make ToolBar shrinked.
Select any button from dropdown (">>" -> and click any).
Click (">>") again

I catch:

java.lang.NullPointerException
at com.sun.javafx.scene.control.skin.ButtonSkin.handleControlPropertyChanged(ButtonSkin.java:91)
at com.sun.javafx.scene.control.skin.SkinBase$3.changed(SkinBase.java:282)
at javafx.beans.value.WeakChangeListener.changed(WeakChangeListener.java:107)
at com.sun.javafx.binding.ExpressionHelper$SingleChange.fireValueChangedEvent(ExpressionHelper.java:196)
at com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(ExpressionHelper.java:100)
at javafx.beans.property.ReadOnlyObjectWrapper$ReadOnlyPropertyImpl.fireValueChangedEvent(ReadOnlyObjectWrapper.java:195)
at javafx.beans.property.ReadOnlyObjectWrapper.fireValueChangedEvent(ReadOnlyObjectWrapper.java:161)
at javafx.beans.property.ObjectPropertyBase.markInvalid(ObjectPropertyBase.java:130)
at javafx.beans.property.ObjectPropertyBase.set(ObjectPropertyBase.java:163)
at javafx.scene.Node.setParent(Node.java:589)
at javafx.scene.Parent$1.onProposedChange(Parent.java:371)
at com.sun.javafx.collections.VetoableObservableList.remove(VetoableObservableList.java:172)
at com.sun.javafx.collections.ObservableListWrapper.remove(ObservableListWrapper.java:263)
at com.sun.javafx.collections.VetoableObservableList.remove(VetoableObservableList.java:179)
at javafx.scene.Parent$1.onChanged(Parent.java:398)
at com.sun.javafx.collections.VetoableObservableList.callObservers(VetoableObservableList.java:83)
at com.sun.javafx.collections.ObservableListWrapper.add(ObservableListWrapper.java:154)
at com.sun.javafx.collections.VetoableObservableList.add(VetoableObservableList.java:167)
at com.sun.javafx.collections.ObservableListWrapper.add(ObservableListWrapper.java:144)
at com.sun.javafx.scene.control.skin.ContextMenuContent$MenuItemContainer.createNodeMenuItemChildren(ContextMenuContent.java:1194)
at com.sun.javafx.scene.control.skin.ContextMenuContent$MenuItemContainer.createChildren(ContextMenuContent.java:1031)
at com.sun.javafx.scene.control.skin.ContextMenuContent$MenuItemContainer.<init>(ContextMenuContent.java:1010)
at com.sun.javafx.scene.control.skin.ContextMenuContent.updateVisualItems(ContextMenuContent.java:207)
at com.sun.javafx.scene.control.skin.ContextMenuContent.access$1700(ContextMenuContent.java:83)
at com.sun.javafx.scene.control.skin.ContextMenuContent$6.onChanged(ContextMenuContent.java:717)
at com.sun.javafx.collections.ListListenerHelper$SingleChange.fireValueChangedEvent(ListListenerHelper.java:134)
at com.sun.javafx.collections.ListListenerHelper.fireValueChangedEvent(ListListenerHelper.java:48)
at com.sun.javafx.collections.ObservableListWrapper.callObservers(ObservableListWrapper.java:97)
at com.sun.javafx.collections.TrackableObservableList.callObservers(TrackableObservableList.java:51)
at com.sun.javafx.collections.ObservableListWrapper.addAll(ObservableListWrapper.java:171)
at com.sun.javafx.collections.ObservableListWrapper.addAll(ObservableListWrapper.java:160)
at com.sun.javafx.scene.control.skin.ToolBarSkin$ToolBarOverflowMenu$1.handle(ToolBarSkin.java:499)
at com.sun.javafx.scene.control.skin.ToolBarSkin$ToolBarOverflowMenu$1.handle(ToolBarSkin.java:493)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:69)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:217)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:170)
at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:38)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:37)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:53)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:33)
at javafx.event.Event.fireEvent(Event.java:171)
at javafx.scene.Scene$MouseHandler.process(Scene.java:3319)
at javafx.scene.Scene$MouseHandler.process(Scene.java:3159)
at javafx.scene.Scene$MouseHandler.access$1900(Scene.java:3114)
at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1536)
at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2256)
at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:227)
at com.sun.glass.ui.View.handleMouseEvent(View.java:528)
at com.sun.glass.ui.View.notifyMouse(View.java:921)
at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at com.sun.glass.ui.win.WinApplication.access$100(WinApplication.java:29)
at com.sun.glass.ui.win.WinApplication$2$1.run(WinApplication.java:62)
at java.lang.Thread.run(Thread.java:662)