Drop a Button.
From the Code section, try switch to "script mode" an event (using the cog icon).
One get the following assertion:

Exception in thread "JavaFX Application Thread" java.lang.AssertionError
at com.oracle.javafx.scenebuilder.kit.editor.panel.inspector.editors.EventHandlerEditor$2.handle(EventHandlerEditor.java:99)
at com.oracle.javafx.scenebuilder.kit.editor.panel.inspector.editors.EventHandlerEditor$2.handle(EventHandlerEditor.java:93)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:49)
at javafx.event.Event.fireEvent(Event.java:203)
at javafx.scene.control.MenuItem.fire(MenuItem.java:462)
at com.sun.javafx.scene.control.skin.ContextMenuContent$MenuItemContainer.doSelect(ContextMenuContent.java:1354)
at com.sun.javafx.scene.control.skin.ContextMenuContent$MenuItemContainer.doSelect(ContextMenuContent.java:1329)
at com.sun.javafx.scene.control.skin.ContextMenuContent$MenuItemContainer$7.handle(ContextMenuContent.java:1297)
at com.sun.javafx.scene.control.skin.ContextMenuContent$MenuItemContainer$7.handle(ContextMenuContent.java:1295)
at com.sun.javafx.event.CompositeEventHandler$NormalEventHandlerRecord.handleBubblingEvent(CompositeEventHandler.java:218)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:80)