Null-check view observer. b=264683 r+sr=roc

git-svn-id: svn://10.0.0.236/trunk@163948 18797224-902f-48f8-a5cc-f745e15eee43

mozilla/view/src/nsViewManager.cpp

@@ -427,7 +427,7 @@ static PRBool IsViewVisible(nsView *aView)
   // content boundaries in DocumentViewerImpl::MakeWindow).
   nsCOMPtr<nsIViewObserver> vo;
   aView->GetViewManager()->GetViewObserver(*getter_AddRefs(vo));
-  return vo->IsVisible();
+  return vo && vo->IsVisible();
 }
 
 void