137815 - crash hitting enter key before panel content finishes loading [@nsTreeSelection::FireOnSelectHandler]. r=bryner sr=ben

layout/xul/base/src/tree/src/nsTreeSelection.cpp

@@ -717,7 +717,12 @@ nsTreeSelection::FireOnSelectHandler()
   nsCOMPtr<nsIContent> content(do_QueryInterface(elt));
   nsCOMPtr<nsIDocument> document;
   content->GetDocument(*getter_AddRefs(document));
- 
+  
+  // we might be firing on a delay, so it's possible in rare cases that
+  // the document may have been destroyed by the time it fires
+  if (!document)
+    return NS_OK;
+
   PRInt32 count = document->GetNumberOfShells();
   for (PRInt32 i = 0; i < count; i++) {
     nsCOMPtr<nsIPresShell> shell;