Assertion error below when trying to drag a Netbeans window from one mode to another. The netbeans window has an embedded JFXPanel, per below.

        fxPanel = new JFXPanel();
        setLayout(new BorderLayout());
        add(fxPanel, BorderLayout.CENTER);

        Platform.runLater(new Runnable() {
            @Override
            public void run() {
                try {
                    Pane conceptDetailsPane = (Pane) FXMLLoader.load(getClass().getResource("/org/ihtsdo/ttk/pl/fx/concept/details/ConceptDetailsView.fxml"));

                    Scene myScene = new Scene(conceptDetailsPane);
                    fxPanel.setScene(myScene);
                    fxPanel.setVisible(true);
                } catch (IOException ex) {
                    Exceptions.printStackTrace(ex);
                }
            }
        });
---------------------------------------

java.lang.AssertionError
at javafx.embed.swing.SwingDnD.endDnD(SwingDnD.java:342)
at javafx.embed.swing.SwingDnD.access$900(SwingDnD.java:71)
at javafx.embed.swing.SwingDnD$3.dragExit(SwingDnD.java:170)
at java.awt.dnd.DropTarget.dragExit(DropTarget.java:426)
at sun.awt.dnd.SunDropTargetContextPeer.processExitMessage(SunDropTargetContextPeer.java:401)
at sun.awt.dnd.SunDropTargetContextPeer$EventDispatcher.dispatchExitEvent(SunDropTargetContextPeer.java:838)
at sun.awt.dnd.SunDropTargetContextPeer$EventDispatcher.dispatchEvent(SunDropTargetContextPeer.java:772)
at sun.awt.dnd.SunDropTargetEvent.dispatch(SunDropTargetEvent.java:48)
at java.awt.Component.dispatchEventImpl(Component.java:4716)
at java.awt.Container.dispatchEventImpl(Container.java:2287)
at java.awt.Component.dispatchEvent(Component.java:4687)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832)
at java.awt.LightweightDispatcher.trackMouseEnterExit(Container.java:4612)
at java.awt.LightweightDispatcher.processDropTargetEvent(Container.java:4558)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4417)
at java.awt.Container.dispatchEventImpl(Container.java:2273)
at java.awt.Window.dispatchEventImpl(Window.java:2719)
at java.awt.Component.dispatchEvent(Component.java:4687)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:729)
at java.awt.EventQueue.access$200(EventQueue.java:103)
at java.awt.EventQueue$3.run(EventQueue.java:688)
at java.awt.EventQueue$3.run(EventQueue.java:686)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
at java.awt.EventQueue$4.run(EventQueue.java:702)
at java.awt.EventQueue$4.run(EventQueue.java:700)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:699)
at org.netbeans.core.TimableEventQueue.dispatchEvent(TimableEventQueue.java:159)
[catch] at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)