Fix bug 269832: crash when scrolling some plugins. Add null-check on mWidget in XP_MACOSX code. r/sr=bz

layout/generic/nsObjectFrame.cpp

@@ -3454,7 +3454,8 @@ nsresult nsPluginInstanceOwner::ScrollPositionDidChange(nsIScrollableView* aScro
 
 #if defined(XP_MACOSX)
       // FIXME - Only invalidate the newly revealed amount.
-      mWidget->Invalidate(PR_TRUE);
+      if (mWidget)
+        mWidget->Invalidate(PR_TRUE);
 #endif
     }