Null-check for unreproducable topcrash. b=267804 r+sr=jst

git-svn-id: svn://10.0.0.236/trunk@165057 18797224-902f-48f8-a5cc-f745e15eee43

mozilla/docshell/base/nsDocShell.cpp

@@ -3342,6 +3342,13 @@ nsDocShell::GetVisibility(PRBool * aVisibility)
         nsCOMPtr<nsIPresShell> pPresShell;
         parentDS->GetPresShell(getter_AddRefs(pPresShell));
 
+        // Null-check for crash in bug 267804
+        if (!pPresShell) {
+            NS_NOTREACHED("docshell has null pres shell");
+            *aVisibility = PR_FALSE;
+            return NS_OK;
+        }
+
         nsIContent *shellContent =
             pPresShell->GetDocument()->FindContentForSubDocument(presShell->GetDocument());
         NS_ASSERTION(shellContent, "subshell not in the map");