I'm encountering a NullPointerException when removing a JFXPanel from a Swing hierarchy. Unfortunately, I'm unable to put together a set of steps to reproduce the issue reliably.

The exception is thrown at line 929 of the JFXPanel class, where the panel's SwingDnD object is dereferenced without a null check. Simply adding a null check seems to fix the issue:

        @Override
        public void setEmbeddedScene(EmbeddedSceneInterface embeddedScene) {
            if (scenePeer == embeddedScene) {
                return;
            }
            scenePeer = embeddedScene;
            if (scenePeer == null) {
                invokeOnClientEDT(() -> {
                    dnd.removeNotify();
                    dnd = null;
                });
                return;
            }
            ...

Should become

        @Override
        public void setEmbeddedScene(EmbeddedSceneInterface embeddedScene) {
            if (scenePeer == embeddedScene) {
                return;
            }
            scenePeer = embeddedScene;
            if (scenePeer == null) {
                invokeOnClientEDT(() -> {
                    if (dnd != null) {
                        dnd.removeNotify();
                    }
                    dnd = null;
                });
                return;
            }



This is the stacktrace:
                            java.lang.NullPointerException
                             at javafx.embed.swing.JFXPanel$HostContainer.lambda$setEmbeddedScene$53(JFXPanel.java:929)
                             at javafx.embed.swing.JFXPanel$HostContainer$$Lambda$18859/13594061.run(Unknown Source)
                             at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:311)
                             at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:749)
                             at java.awt.EventQueue.access$500(EventQueue.java:97)
                             at java.awt.EventQueue$3.run(EventQueue.java:702)
                             at java.awt.EventQueue$3.run(EventQueue.java:696)
                             at java.security.AccessController.doPrivileged(Native Method)
                             at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:75)
                             at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:86)
                             at java.awt.EventQueue$4.run(EventQueue.java:724)
                             at java.awt.EventQueue$4.run(EventQueue.java:722)
                             at java.security.AccessController.doPrivileged(Native Method)
                             at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:75)
                             at java.awt.EventQueue.dispatchEvent(EventQueue.java:721)
                             at jSSS.jgui.sssworker.SSSMouseBlocker.dispatchEvent(SSSMouseBlocker.java:105)
                             at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
                             at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
                             at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
                             at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
                             at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
                             at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)

The call into invokeOnClientEDT() has the following stacktrace:
at javafx.embed.swing.JFXPanel$HostContainer.setEmbeddedScene(JFXPanel.java:928)
at com.sun.javafx.tk.quantum.EmbeddedScene.setStage(EmbeddedScene.java:100)
at com.sun.javafx.tk.quantum.GlassStage.setScene(GlassStage.java:96)
at com.sun.javafx.tk.quantum.EmbeddedStage.setScene(EmbeddedStage.java:63)
at javafx.stage.Window$9.invalidated(Window.java:866)
at javafx.beans.property.BooleanPropertyBase.markInvalid(BooleanPropertyBase.java:109)
at javafx.beans.property.BooleanPropertyBase.set(BooleanPropertyBase.java:144)
at javafx.stage.Window.setShowing(Window.java:910)
at javafx.stage.Window.hide(Window.java:935)
at javafx.embed.swing.JFXPanel.lambda$removeNotify$50(JFXPanel.java:869)
at javafx.embed.swing.JFXPanel$$Lambda$18857.29065563.run(Unknown Source:-1)
at com.sun.javafx.application.PlatformImpl.lambda$null$170(PlatformImpl.java:295)
at com.sun.javafx.application.PlatformImpl$$Lambda$18601.12763758.run(Unknown Source:-1)
at java.security.AccessController.doPrivileged(AccessController.java:-1)
at com.sun.javafx.application.PlatformImpl.lambda$runLater$171(PlatformImpl.java:294)
at com.sun.javafx.application.PlatformImpl$$Lambda$18598.18016287.run(Unknown Source:-1)
at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)
at com.sun.glass.ui.win.WinApplication._runLoop(WinApplication.java:-1)
at com.sun.glass.ui.win.WinApplication.lambda$null$145(WinApplication.java:101)
at com.sun.glass.ui.win.WinApplication$$Lambda$18587.12630745.run(Unknown Source:-1)
at java.lang.Thread.run(Thread.java:745)

This issue has already been raised as JMY-284, but that appears to be the wrong project for issues like this.

Note also that this defect was not present in JDK 1.8.0_31.