Bug 348197 firefox crash with orca running [@nsAccessNode::GetDocAccessibleFor]

patch by Evan.Yan at sun.com r=aaronleventhal

accessible/src/base/nsRootAccessible.cpp

@@ -985,7 +985,11 @@ NS_IMETHODIMP nsRootAccessible::GetAccessibleRelated(PRUint32 aRelationType,
 
   nsCOMPtr<nsIDocShellTreeItem> treeItem = GetDocShellTreeItemFor(mDOMNode);   
   nsCOMPtr<nsIDocShellTreeItem> contentTreeItem = GetContentDocShell(treeItem);
+  // there may be no content area, so we need a null check
+  if (contentTreeItem) {
     nsCOMPtr<nsIAccessibleDocument> accDoc = GetDocAccessibleFor(contentTreeItem);
     return accDoc->QueryInterface(NS_GET_IID(nsIAccessible), (void**)aRelated);
   }
+  return NS_OK;
+}