Make GetCurrentEventFrame() return null if the shell is being destroyed (don't call GetPrimaryFrameFor()). b=421839 r+sr=roc

layout/base/nsPresShell.cpp

@@ -5448,6 +5448,10 @@ PresShell::Paint(nsIView*             aView,
 nsIFrame*
 PresShell::GetCurrentEventFrame()
 {
+  if (NS_UNLIKELY(mIsDestroying)) {
+    return nsnull;
+  }
+    
   if (!mCurrentEventFrame && mCurrentEventContent) {
     // Make sure the content still has a document reference. If not,
     // then we assume it is no longer in the content tree and the