In noticed this issue while testing JavaFX on Linux. This issue happens on both pipes, ie. j2d or es2. The steps to reproduce this bug is as followed:

1) Launch Ensemble
2) Select Stage->Advanced Stage
3) Click on the "Create a Stage" button
4) Click on the "minimalise me" button in the newly created Stage
5) Click on the "X" to exit Ensemble. You should now see the following stack trace:

JavaFX Application Thread uncaught: java.lang.RuntimeException
java.lang.RuntimeException: WindowStage.setIconified failed
at com.sun.javafx.tk.quantum.WindowStage.setIconified(WindowStage.java:442)
at javafx.stage.Stage$6.invalidated(Stage.java:559)
at javafx.beans.property.BooleanPropertyBase.markInvalid(BooleanPropertyBase.java:129)
at javafx.beans.property.BooleanPropertyBase.set(BooleanPropertyBase.java:163)
at javafx.stage.Stage.setIconified(Stage.java:541)
at ensemble.samples.scenegraph.stage.AdvancedStageSample$1$4.handle(AdvancedStageSample.java:145)
at ensemble.samples.scenegraph.stage.AdvancedStageSample$1$4.handle(AdvancedStageSample.java:143)
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.EventUtil.fireEventImpl(EventUtil.java:53)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:28)
at javafx.event.Event.fireEvent(Event.java:171)
at javafx.scene.Node.fireEvent(Node.java:5975)
at javafx.scene.control.Button.fire(Button.java:179)
at com.sun.javafx.scene.control.behavior.ButtonBehavior.mouseReleased(ButtonBehavior.java:189)
at com.sun.javafx.scene.control.skin.SkinBase$4.handle(SkinBase.java:336)
at com.sun.javafx.scene.control.skin.SkinBase$4.handle(SkinBase.java:329)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:64)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:217)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:170)
at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:38)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:37)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:35)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:53)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:33)
at javafx.event.Event.fireEvent(Event.java:171)
at javafx.scene.Scene$MouseHandler.process(Scene.java:2897)
at javafx.scene.Scene$MouseHandler.process(Scene.java:2691)
at javafx.scene.Scene$MouseHandler.access$1300(Scene.java:2657)
at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1281)
at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:1915)
at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:215)
at com.sun.glass.ui.View.handleMouseEvent(View.java:493)
at com.sun.glass.ui.View.notifyMouse(View.java:871)
at com.sun.glass.ui.gtk.GtkApplication._runLoop(Native Method)
at com.sun.glass.ui.gtk.GtkApplication$2$1.run(GtkApplication.java:79)
at java.lang.Thread.run(Thread.java:662)