While testing SceneBuilder on Linux, I twice (with two different runs) got the following exception. No steps to reproduce this since it is intermittent.

BuilderUncaughtExceptionHandler uncaughtException
SEVERE: An exception was thrown:
java.lang.NullPointerException
at java.util.concurrent.ConcurrentHashMap.get(ConcurrentHashMap.java:936)
at java.util.concurrent.ConcurrentHashMap.containsKey(ConcurrentHashMap.java:964)
at com.sun.javafx.collections.ObservableMapWrapper.remove(ObservableMapWrapper.java:181)
at com.sun.javafx.scene.control.ControlAcceleratorSupport.removeAcceleratorsFromScene(ControlAcceleratorSupport.java:226)
at com.sun.javafx.scene.control.ControlAcceleratorSupport.lambda$doAcceleratorInstall$11(ControlAcceleratorSupport.java:122)
at com.sun.javafx.scene.control.ControlAcceleratorSupport$$Lambda$321/323284542.onChanged(Unknown Source)
at com.sun.javafx.collections.ListListenerHelper$Generic.fireValueChangedEvent(ListListenerHelper.java:329)
at com.sun.javafx.collections.ListListenerHelper.fireValueChangedEvent(ListListenerHelper.java:73)
at javafx.collections.ObservableListBase.fireChange(ObservableListBase.java:233)
at javafx.collections.ListChangeBuilder.commit(ListChangeBuilder.java:482)
at javafx.collections.ListChangeBuilder.endChange(ListChangeBuilder.java:541)
at javafx.collections.ObservableListBase.endChange(ObservableListBase.java:205)
at com.sun.javafx.collections.ObservableListWrapper.removeAll(ObservableListWrapper.java:185)
at com.sun.javafx.scene.control.skin.MenuBarSkin.lambda$rebuildUI$381(MenuBarSkin.java:621)
at com.sun.javafx.scene.control.skin.MenuBarSkin$$Lambda$306/537635508.onChanged(Unknown Source)
at com.sun.javafx.collections.ListListenerHelper$Generic.fireValueChangedEvent(ListListenerHelper.java:329)
at com.sun.javafx.collections.ListListenerHelper.fireValueChangedEvent(ListListenerHelper.java:73)
at javafx.collections.ObservableListBase.fireChange(ObservableListBase.java:233)
at javafx.collections.ListChangeBuilder.commit(ListChangeBuilder.java:482)
at javafx.collections.ListChangeBuilder.endChange(ListChangeBuilder.java:541)
at javafx.collections.ObservableListBase.endChange(ObservableListBase.java:205)
at com.sun.javafx.collections.ObservableListWrapper.clear(ObservableListWrapper.java:157)
at com.oracle.javafx.scenebuilder.app.menubar.MenuBarController.handleOnWindowMenuValidation(MenuBarController.java:1361)
at com.oracle.javafx.scenebuilder.app.menubar.MenuBarController.lambda$new$22(MenuBarController.java:1357)
at com.oracle.javafx.scenebuilder.app.menubar.MenuBarController$$Lambda$79/1769450488.handle(Unknown Source)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:49)
at javafx.event.Event.fireEvent(Event.java:198)
at javafx.scene.control.Menu.setShowing(Menu.java:176)
at javafx.scene.control.Menu.show(Menu.java:385)
at com.sun.javafx.scene.control.skin.MenuBarSkin.lambda$rebuildUI$387(MenuBarSkin.java:676)
at com.sun.javafx.scene.control.skin.MenuBarSkin$$Lambda$312/866770695.handle(Unknown Source)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)
at javafx.event.Event.fireEvent(Event.java:198)
at javafx.scene.Scene$MouseHandler.process(Scene.java:3719)
at javafx.scene.Scene$MouseHandler.access$1500(Scene.java:3447)
at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1723)
at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2456)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:348)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:273)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:382)
at com.sun.glass.ui.View.handleMouseEvent(View.java:549)
at com.sun.glass.ui.View.notifyMouse(View.java:921)
at com.sun.glass.ui.gtk.GtkApplication._runLoop(Native Method)
at com.sun.glass.ui.gtk.GtkApplication.lambda$null$45(GtkApplication.java:126)
at com.sun.glass.ui.gtk.GtkApplication$$Lambda$42/1531448569.run(Unknown Source)
at java.lang.Thread.run(Thread.java:744)