We have a Webview in a JFXPanel. Drag and drop in the webview throws exceptions.

Steps to reproduce:
1. Select text, or find an embedded image.
2. Drag and drop the selected text or image.

This is similar to RT-19715.

Exception in runnable
java.lang.IllegalArgumentException: Key already associated with a running event loop: java.lang.Object@f0896b1
at com.sun.javafx.tk.quantum.QuantumToolkit.enterNestedEventLoop(Unknown Source)
at com.sun.javafx.tk.quantum.FxEventLoop$Impl.start(Unknown Source)
at com.sun.javafx.tk.quantum.FxEventLoop.enterNestedLoop(Unknown Source)
at com.sun.javafx.tk.quantum.EmbeddedSceneDnD$ClipboardAssistanceImpl.flush(Unknown Source)
at com.sun.javafx.tk.quantum.QuantumClipboard.flush(Unknown Source)
at com.sun.javafx.tk.quantum.QuantumToolkit.startDrag(Unknown Source)
at javafx.scene.Scene$DnDGesture.dragDetectedProcessed(Unknown Source)
at javafx.scene.Scene$DnDGesture.process(Unknown Source)
at javafx.scene.Scene$DnDGesture.access$8700(Unknown Source)
at javafx.scene.Scene$MouseHandler.process(Unknown Source)
at javafx.scene.Scene$MouseHandler.process(Unknown Source)
at javafx.scene.Scene$MouseHandler.access$1900(Unknown Source)
at javafx.scene.Scene.impl_processMouseEvent(Unknown Source)
at javafx.scene.Scene$ScenePeerListener.mouseEvent(Unknown Source)
at com.sun.javafx.tk.quantum.EmbeddedScene$2.run(Unknown Source)
at com.sun.javafx.application.PlatformImpl$4.run(Unknown Source)
at com.sun.glass.ui.win.WinApplication._enterNestedEventLoop(Native Method)
at com.sun.glass.ui.Application.enterNestedEventLoop(Unknown Source)
at com.sun.glass.ui.EventLoop.enter(Unknown Source)
at com.sun.javafx.tk.quantum.QuantumToolkit.enterNestedEventLoop(Unknown Source)
at com.sun.javafx.tk.quantum.FxEventLoop$Impl.start(Unknown Source)
at com.sun.javafx.tk.quantum.FxEventLoop.enterNestedLoop(Unknown Source)
at com.sun.javafx.tk.quantum.EmbeddedSceneDnD$ClipboardAssistanceImpl.flush(Unknown Source)
at com.sun.javafx.tk.quantum.QuantumClipboard.flush(Unknown Source)
at com.sun.javafx.tk.quantum.QuantumToolkit.startDrag(Unknown Source)
at javafx.scene.Scene$DnDGesture.dragDetectedProcessed(Unknown Source)
at javafx.scene.Scene$DnDGesture.process(Unknown Source)
at javafx.scene.Scene$DnDGesture.access$8700(Unknown Source)
at javafx.scene.Scene$MouseHandler.process(Unknown Source)
at javafx.scene.Scene$MouseHandler.process(Unknown Source)
at javafx.scene.Scene$MouseHandler.access$1900(Unknown Source)
at javafx.scene.Scene.impl_processMouseEvent(Unknown Source)
at javafx.scene.Scene$ScenePeerListener.mouseEvent(Unknown Source)
at com.sun.javafx.tk.quantum.EmbeddedScene$2.run(Unknown Source)
at com.sun.javafx.application.PlatformImpl$4.run(Unknown Source)
at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at com.sun.glass.ui.win.WinApplication.access$100(Unknown Source)
at com.sun.glass.ui.win.WinApplication$2$1.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at javafx.embed.swing.SwingDnD$2.dragDropEnd(Unknown Source)
at java.awt.dnd.DnDEventMulticaster.dragDropEnd(Unknown Source)
at java.awt.dnd.DragSource.processDragDropEnd(Unknown Source)
at java.awt.dnd.DragSourceContext.dragDropEnd(Unknown Source)
at sun.awt.dnd.SunDragSourceContextPeer$EventDispatcher.run(Unknown Source)
at java.awt.event.InvocationEvent.dispatch(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$000(Unknown Source)
at java.awt.EventQueue$1.run(Unknown Source)
at java.awt.EventQueue$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source)
at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue$2.run(Unknown Source)
at java.awt.EventQueue$2.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)