To reproduce add menubar onto content panel.
Try to remove default menus.
Try to add menus

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com.oracle.javafx.authoring.model.transactions.Transaction$TransactionFailed: transaction.drop.on.stage failed: java.lang.IllegalArgumentException: Failed to set javafx.scene.control.MenuBar.menus to [javafx.scene.control.Menu@12eb79]
at com.oracle.javafx.authoring.model.transactions.Transaction.register(Transaction.java:181)
at com.oracle.javafx.authoring.model.transactions.Transaction.addItemToList(Transaction.java:557)
at com.oracle.javafx.authoring.dictionary.definition.MenuBarComponentDefinition.addChild(MenuBarComponentDefinition.java:90)
at com.oracle.javafx.authoring.stage.v2.DropOperation.run(DropOperation.java:142)
at com.oracle.javafx.authoring.stage.v2.NewComponentDndGesture.drop(NewComponentDndGesture.java:81)
at com.oracle.javafx.authoring.stage.v2.EditModeController.drop(EditModeController.java:290)
at com.oracle.javafx.authoring.StageView.drop(StageView.java:631)
at com.oracle.javafx.authoring.controls.TreeDragContext.finish(TreeDragContext.java:187)
at com.oracle.javafx.authoring.controls.TreeDragContext.finish(TreeDragContext.java:169)
at com.oracle.javafx.authoring.panels.library.LibraryDragSupport.handleMouseRelease(LibraryDragSupport.java:73)
at com.oracle.javafx.authoring.panels.library.LibraryPanel$2.handle(LibraryPanel.java:102)
at com.oracle.javafx.authoring.panels.library.LibraryPanel$2.handle(LibraryPanel.java:98)
at com.sun.javafx.event.CompositeEventHandler.dispatchCapturingEvent(CompositeEventHandler.java:78)
at com.sun.javafx.event.EventHandlerManager.dispatchCapturingEvent(EventHandlerManager.java:202)
at com.sun.javafx.event.EventHandlerManager.dispatchCapturingEvent(EventHandlerManager.java:159)
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.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.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:3285)
at javafx.scene.Scene$MouseHandler.process(Scene.java:3125)
at javafx.scene.Scene$MouseHandler.access$1900(Scene.java:3080)
at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1507)
at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2223)
at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:227)
at com.sun.glass.ui.View.handleMouseEvent(View.java:523)
at com.sun.glass.ui.View.notifyMouse(View.java:916)
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$1.run(WinApplication.java:62)
at java.lang.Thread.run(Thread.java:722)
Caused by: java.lang.IllegalArgumentException: Failed to set javafx.scene.control.MenuBar.menus to [javafx.scene.control.Menu@12eb79]
at com.oracle.javafx.authoring.persist.Prop$BeanProp.set(Prop.java:111)
at com.oracle.javafx.authoring.model.transactions.SetPropertyAction.apply(SetPropertyAction.java:107)
at com.oracle.javafx.authoring.model.transactions.Transaction.register(Transaction.java:173)
... 46 more