With assertions enabled you get the one below when loading the attached layout.

Exception in thread "AppKit Thread" java.lang.AssertionError
at com.oracle.javafx.scenebuilder.kit.fxom.FXOMLoader.readPropertyAttribute(FXOMLoader.java:234)
at javafx.fxml.FXMLLoader$Element.processAttribute(FXMLLoader.java:252)
at javafx.fxml.FXMLLoader$PropertyElement.processAttribute(FXMLLoader.java:1426)
at javafx.fxml.FXMLLoader$Element.processStartElement(FXMLLoader.java:216)
at javafx.fxml.FXMLLoader.processStartElement(FXMLLoader.java:2723)
at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2527)
at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2425)
at com.oracle.javafx.scenebuilder.kit.fxom.FXOMLoader.load(FXOMLoader.java:89)
at com.oracle.javafx.scenebuilder.kit.fxom.FXOMDocument.<init>(FXOMDocument.java:80)
at com.oracle.javafx.scenebuilder.kit.editor.EditorController.updateFxomDocument(EditorController.java:2283)
at com.oracle.javafx.scenebuilder.kit.editor.EditorController.setFxmlTextAndLocation(EditorController.java:615)
at com.oracle.javafx.scenebuilder.app.DocumentWindowController.loadFromFile(DocumentWindowController.java:393)
at com.oracle.javafx.scenebuilder.app.SceneBuilderApp.performOpenFiles(SceneBuilderApp.java:484)
at com.oracle.javafx.scenebuilder.app.SceneBuilderApp.performOpenRecent(SceneBuilderApp.java:191)
at com.oracle.javafx.scenebuilder.app.menubar.MenuBarController$21.handle(MenuBarController.java:1126)
at com.oracle.javafx.scenebuilder.app.menubar.MenuBarController$21.handle(MenuBarController.java:1121)
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:204)
at javafx.scene.control.MenuItem.fire(MenuItem.java:462)
at com.sun.javafx.scene.control.GlobalMenuAdapter$5.handle(GlobalMenuAdapter.java:158)
at com.sun.javafx.scene.control.GlobalMenuAdapter$5.handle(GlobalMenuAdapter.java:156)
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:204)
at javafx.scene.control.MenuItem.fire(MenuItem.java:462)
at com.sun.javafx.tk.quantum.GlassSystemMenu$6.action(GlassSystemMenu.java:234)