Bug 1157195, null check for widget before generating drag events, r=masayuki

dom/events/EventStateManager.cpp

@@ -1589,7 +1589,7 @@ EventStateManager::GenerateDragGesture(nsPresContext* aPresContext,
   if (IsTrackingDragGesture()) {
     mCurrentTarget = mGestureDownFrameOwner->GetPrimaryFrame();
 
-    if (!mCurrentTarget) {
+    if (!mCurrentTarget || !mCurrentTarget->GetNearestWidget()) {
       StopTrackingDragGesture();
       return;
     }