Drag and drop null source


Consider this code:

final DragAndDropWrapper dropTarget = new DragAndDropWrapper(root);
		dropTarget.setDropHandler(new DropHandler() {
			public void drop(DragAndDropEvent event) {
				try {
					if (event == null) handleNullPointer("event");
					if (event.getTransferable() == null) handleNullPointer("event.getTransferable()");
					if (event.getTransferable().getSourceComponent() == null) handleNullPointer("event.getTransferable().getSourceComponent()");

The last line is occasionally triggering the null handler. What are the circumstances that could result in event.getTransferable().getSourceComponent() returning null? I can’t repeat it and it doesn’t appear my clients can either. It just happens once in a while. Could it be a weak reference of some kind?