Tried to open Unlock.fxml from the UnlockInclude sample.
Got the following stack:

Exception in thread "JavaFX Application Thread" java.lang.AssertionError
at com.oracle.javafx.scenebuilder.kit.fxom.FXOMLoader.readPropertyAttribute(FXOMLoader.java:197)
at javafx.fxml.FXMLLoader$Element.processAttribute(FXMLLoader.java:246)
at javafx.fxml.FXMLLoader$ValueElement.processAttribute(FXMLLoader.java:931)
at javafx.fxml.FXMLLoader$IncludeElement.processAttribute(FXMLLoader.java:1086)
at javafx.fxml.FXMLLoader$Element.processStartElement(FXMLLoader.java:210)
at javafx.fxml.FXMLLoader$ValueElement.processStartElement(FXMLLoader.java:724)
at javafx.fxml.FXMLLoader.processStartElement(FXMLLoader.java:2578)
at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2390)
at com.oracle.javafx.scenebuilder.kit.fxom.FXOMLoader.load(FXOMLoader.java:53)
at com.oracle.javafx.scenebuilder.kit.fxom.FXOMDocument.<init>(FXOMDocument.java:41)
at com.oracle.javafx.scenebuilder.kit.editor.EditorController.updateFxomDocument(EditorController.java:1005)
at com.oracle.javafx.scenebuilder.kit.editor.EditorController.setFxmlTextAndLocation(EditorController.java:319)
at com.oracle.javafx.scenebuilder.app.DocumentWindowController.loadFromFile(DocumentWindowController.java:143)
at com.oracle.javafx.scenebuilder.app.SceneBuilderApp.performOpenFile(SceneBuilderApp.java:230)
at com.oracle.javafx.scenebuilder.app.SceneBuilderApp.performControlAction(SceneBuilderApp.java:65)
at com.oracle.javafx.scenebuilder.app.menubar.MenuBarController$ApplicationControlActionController.perform(MenuBarController.java:903)
at com.oracle.javafx.scenebuilder.app.menubar.MenuBarController.handleOnActionMenu(MenuBarController.java:612)
at com.oracle.javafx.scenebuilder.app.menubar.MenuBarController.access$300(MenuBarController.java:48)
at com.oracle.javafx.scenebuilder.app.menubar.MenuBarController$8.handle(MenuBarController.java:605)
at com.oracle.javafx.scenebuilder.app.menubar.MenuBarController$8.handle(MenuBarController.java:601)
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.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:203)
at javafx.scene.control.MenuItem.fire(MenuItem.java:462)
at com.sun.javafx.tk.quantum.GlassSystemMenu$6.action(GlassSystemMenu.java:234)