Open attached fxml.
Select Button and Label.
Invoke Wrap In ToolBar (either menu or shortcut).
Invoke UNDO (either menu or shortcut).

You will get following exception :
java.lang.AssertionError
at com.oracle.javafx.scenebuilder.kit.fxom.FXOMRefresher.refreshFxomInstance(FXOMRefresher.java:143)
at com.oracle.javafx.scenebuilder.kit.fxom.FXOMRefresher.refreshFxomObject(FXOMRefresher.java:120)
at com.oracle.javafx.scenebuilder.kit.fxom.FXOMRefresher.refreshFxomObjects(FXOMRefresher.java:195)
at com.oracle.javafx.scenebuilder.kit.fxom.FXOMRefresher.refreshFxomProperty(FXOMRefresher.java:182)
at com.oracle.javafx.scenebuilder.kit.fxom.FXOMRefresher.refreshFxomInstance(FXOMRefresher.java:147)
at com.oracle.javafx.scenebuilder.kit.fxom.FXOMRefresher.refreshFxomObject(FXOMRefresher.java:120)
at com.oracle.javafx.scenebuilder.kit.fxom.FXOMRefresher.refreshDocument(FXOMRefresher.java:107)
at com.oracle.javafx.scenebuilder.kit.fxom.FXOMRefresher.refresh(FXOMRefresher.java:64)
at com.oracle.javafx.scenebuilder.kit.fxom.FXOMDocument.refreshSceneGraph(FXOMDocument.java:276)
at com.oracle.javafx.scenebuilder.kit.fxom.FXOMDocument.endUpdate(FXOMDocument.java:263)
at com.oracle.javafx.scenebuilder.kit.fxom.FXOMNodes.newDocument(FXOMNodes.java:267)
at com.oracle.javafx.scenebuilder.kit.editor.job.DuplicateSelectionJob.makeSubJobs(DuplicateSelectionJob.java:79)
at com.oracle.javafx.scenebuilder.kit.editor.job.BatchDocumentJob.getSubJobs(BatchDocumentJob.java:56)
at com.oracle.javafx.scenebuilder.kit.editor.job.BatchDocumentJob.isExecutable(BatchDocumentJob.java:64)
at com.oracle.javafx.scenebuilder.kit.editor.EditorController.canPerformEditAction(EditorController.java:1206)
at com.oracle.javafx.scenebuilder.app.menubar.MenuBarController$EditActionController.canPerform(MenuBarController.java:1556)
at com.oracle.javafx.scenebuilder.app.menubar.MenuBarController.handleOnMenuValidation(MenuBarController.java:1068)
at com.oracle.javafx.scenebuilder.app.menubar.MenuBarController.lambda$new$16(MenuBarController.java:1057)
at com.oracle.javafx.scenebuilder.app.menubar.MenuBarController$$Lambda$80/1564944316.handle(Unknown Source)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)
at javafx.event.Event.fireEvent(Event.java:198)
at com.sun.javafx.scene.control.GlobalMenuAdapter$MenuItemAdapter.fireValidation(GlobalMenuAdapter.java:187)
at com.sun.javafx.tk.quantum.GlassSystemMenu$1.validate(GlassSystemMenu.java:243)