java.lang.RuntimeException: A bound value cannot be set.
        at javafx.beans.property.BooleanPropertyBase.set(BooleanPropertyBase.java:118)
        at com.javafx.preview.control.ComboBoxBase.hide(ComboBoxBase.java:214)
        at com.sun.javafx.scene.control.behavior.ComboBoxBaseBehavior.hide(ComboBoxBaseBehavior.java:218)
        at com.sun.javafx.scene.control.behavior.ComboBoxBaseBehavior.onAutoHide(ComboBoxBaseBehavior.java:230)
        at com.sun.javafx.scene.control.skin.ComboBoxPopupControl$2.handle(ComboBoxPopupControl.java:101)
        at javafx.stage.PopupWindow.doAutoHide(PopupWindow.java:370)
        at com.sun.javafx.stage.PopupEventRedirector.handleMousePressedEvent(PopupEventRedirector.java:101)
        at com.sun.javafx.stage.PopupEventRedirector.handleRedirectedEvent(PopupEventRedirector.java:47)
        at com.sun.javafx.event.EventRedirector.dispatchCapturingEvent(EventRedirector.java:85)
        at com.sun.javafx.event.CompositeEventDispatcher.dispatchCapturingEvent(CompositeEventDispatcher.java:22)
        at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:31)
        at com.sun.javafx.event.EventRedirector.redirectEvent(EventRedirector.java:103)
        at com.sun.javafx.event.EventRedirector.dispatchCapturingEvent(EventRedirector.java:82)
        at com.sun.javafx.event.CompositeEventDispatcher.dispatchCapturingEvent(CompositeEventDispatcher.java:22)
        at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:31)
        at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
        at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:53)
        at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:33)
        at javafx.event.Event.fireEvent(Event.java:171)
        at javafx.scene.Scene$MouseHandler.process(Scene.java:2893)
        at javafx.scene.Scene$MouseHandler.process(Scene.java:2687)
        at javafx.scene.Scene$MouseHandler.access$1300(Scene.java:2653)
        at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1279)
        at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:1911)
        at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:204)
        at com.sun.glass.ui.View.handleMouseEvent(View.java:314)
        at com.sun.glass.ui.View.notifyMouse(View.java:639)
        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:722)


import com.javafx.preview.control.ComboBox;
import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.ToggleButton;
import javafx.scene.layout.Pane;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

public class Issue1 extends Application {

    public static void main(String[] args) {
        launch(args);
    }
    final ComboBox testedComboBox = new ComboBox();

    @Override
    public void start(Stage stage) throws Exception {
        Pane pane = new Pane();
        pane.setPrefHeight(200);
        pane.setPrefWidth(200);

        pane.getChildren().add(testedComboBox);

        VBox vb = new VBox();

        ToggleButton toggleButton = new ToggleButton();
        testedComboBox.showingProperty().bind(toggleButton.selectedProperty());

        vb.getChildren().addAll(pane, toggleButton);
        Scene scene = new Scene(vb, 400, 400);
        stage.setScene(scene);
        stage.show();
    }
}

Situation:
If you click on toggle button, you can see, that it controls showing property of comboBox.

Steps to reproduce:
Try to click on comboBox (to open or to hide it).