Bug 1677194 - Avoid accessing invalidated frame. r=emilio

MANUAL PUSH: Author doesn't use Phabricator.

layout/xul/tree/nsTreeBodyFrame.cpp

@@ -1598,10 +1598,17 @@ nsresult nsTreeBodyFrame::RowCountChanged(int32_t aIndex, int32_t aCount) {
   }
 #endif  // #ifdef ACCESSIBILITY
 
+  AutoWeakFrame weakFrame(this);
+
   // Adjust our selection.
+  nsCOMPtr<nsITreeView> view = mView;
   nsCOMPtr<nsITreeSelection> sel;
-  mView->GetSelection(getter_AddRefs(sel));
-  if (sel) sel->AdjustSelection(aIndex, aCount);
+  view->GetSelection(getter_AddRefs(sel));
+  if (sel) {
+    sel->AdjustSelection(aIndex, aCount);
+  }
+
+  NS_ENSURE_STATE(weakFrame.IsAlive());
 
   if (mUpdateBatchNest) return NS_OK;