When dropping on Content the attached fxml file you get the stack below:

Exception in thread "JavaFX Application Thread" java.lang.IllegalStateException: Bug in FXOMRefresher
    at com.oracle.javafx.scenebuilder.kit.fxom.FXOMRefresher.refresh(FXOMRefresher.java:57)
    at com.oracle.javafx.scenebuilder.kit.fxom.FXOMDocument.refreshSceneGraph(FXOMDocument.java:208)
    at com.oracle.javafx.scenebuilder.kit.fxom.FXOMDocument.endUpdate(FXOMDocument.java:195)
    at com.oracle.javafx.scenebuilder.kit.editor.job.BatchJob.execute(BatchJob.java:108)
    at com.oracle.javafx.scenebuilder.kit.editor.job.InsertAsSubComponentJob.execute(InsertAsSubComponentJob.java:147)
    at com.oracle.javafx.scenebuilder.kit.editor.JobManager.executeJob(JobManager.java:160)
    at com.oracle.javafx.scenebuilder.kit.editor.JobManager.push(JobManager.java:66)
    at com.oracle.javafx.scenebuilder.kit.editor.EditorController.performInsert(EditorController.java:1067)
    at com.oracle.javafx.scenebuilder.kit.editor.panel.library.LibraryPanelController.handleKeyEvent(LibraryPanelController.java:509)
    at com.oracle.javafx.scenebuilder.kit.editor.panel.library.LibraryPanelController.access$000(LibraryPanelController.java:93)
    at com.oracle.javafx.scenebuilder.kit.editor.panel.library.LibraryPanelController$3.handle(LibraryPanelController.java:481)
    at com.oracle.javafx.scenebuilder.kit.editor.panel.library.LibraryPanelController$3.handle(LibraryPanelController.java:478)
    at com.sun.javafx.event.CompositeEventHandler$NormalEventHandlerRecord.handleBubblingEvent(CompositeEventHandler.java:218)
    at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:80)
    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.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
    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.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    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.Scene$KeyHandler.process(Scene.java:3862)
    at javafx.scene.Scene$KeyHandler.access$2100(Scene.java:3809)
    at javafx.scene.Scene.impl_processKeyEvent(Scene.java:1993)
    at javafx.scene.Scene$ScenePeerListener.keyEvent(Scene.java:2450)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler$KeyEventNotification.run(GlassViewEventHandler.java:170)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler$KeyEventNotification.run(GlassViewEventHandler.java:123)
    at java.security.AccessController.doPrivileged(Native Method)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleKeyEvent(GlassViewEventHandler.java:197)
    at com.sun.glass.ui.View.handleKeyEvent(View.java:517)
    at com.sun.glass.ui.View.notifyKey(View.java:927)
Caused by: java.io.IOException: javafx.fxml.LoadException: fx:controller can only be applied to root element.
unknown path:8

    at com.oracle.javafx.scenebuilder.kit.fxom.FXOMLoader.load(FXOMLoader.java:84)
    at com.oracle.javafx.scenebuilder.kit.fxom.FXOMDocument.<init>(FXOMDocument.java:71)
    at com.oracle.javafx.scenebuilder.kit.fxom.FXOMRefresher.refresh(FXOMRefresher.java:54)
    ... 47 more
Caused by: javafx.fxml.LoadException: fx:controller can only be applied to root element.
unknown path:8

    at javafx.fxml.FXMLLoader.constructLoadException(FXMLLoader.java:2585)
    at javafx.fxml.FXMLLoader.access$100(FXMLLoader.java:99)
    at javafx.fxml.FXMLLoader$ValueElement.processAttribute(FXMLLoader.java:909)
    at javafx.fxml.FXMLLoader$InstanceDeclarationElement.processAttribute(FXMLLoader.java:971)
    at javafx.fxml.FXMLLoader$Element.processStartElement(FXMLLoader.java:211)
    at javafx.fxml.FXMLLoader$ValueElement.processStartElement(FXMLLoader.java:733)
    at javafx.fxml.FXMLLoader.processStartElement(FXMLLoader.java:2695)
    at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2499)
    at javafx.fxml.FXMLLoader.load(FXMLLoader.java:2397)
    at com.oracle.javafx.scenebuilder.kit.fxom.FXOMLoader.load(FXOMLoader.java:81)
    ... 49 more