UpdateWidget() can fire beforeload event synchronously blowing away RenderArena and its associated …

Source/core/dom/Document.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2011, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2008, 2009, 2011, 2012 Google Inc. All rights reserved.
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) Research In Motion Limited 2010-2011. All rights reserved.
@@ skipping 2345 matching lines @@
         if (view() && renderObject && (!renderObject->firstChild() || renderObject->needsLayout()))
             view()->layout();
     }
 
     m_processingLoadEvent = false;
 
     if (f && renderObject && AXObjectCache::accessibilityEnabled()) {
         // The AX cache may have been cleared at this point, but we need to make sure it contains an
         // AX object to send the notification to. getOrCreate will make sure that an valid AX object
         // exists in the cache (we ignore the return value because we don't need it here). This is 
-        // only safe to call when a layout is not in progress, so it can not be used in postNotification.    
-        axObjectCache()->getOrCreate(renderObject);
-        if (this == topDocument())
-            axObjectCache()->postNotification(renderObject, AXObjectCache::AXLoadComplete, true);
-        else {
-            // AXLoadComplete can only be posted on the top document, so if it's a document
-            // in an iframe that just finished loading, post AXLayoutComplete instead.
-            axObjectCache()->postNotification(renderObject, AXObjectCache::AXLayoutComplete, true);
+        // only safe to call when a layout is not in progress, so it can not be used in postNotification.
+        if (AXObjectCache* cache = axObjectCache()) {
+            cache->getOrCreate(renderObject);
+            if (this == topDocument())
+                cache->postNotification(renderObject, AXObjectCache::AXLoadComplete, true);
+            else {
+                // AXLoadComplete can only be posted on the top document, so if it's a document
+                // in an iframe that just finished loading, post AXLayoutComplete instead.
+                cache->postNotification(renderObject, AXObjectCache::AXLayoutComplete, true);
+            }
         }
     }
 
 #if ENABLE(SVG)
     if (svgExtensions())
         accessSVGExtensions()->startAnimations();
 #endif
 }
 
 void Document::setParsing(bool b)
@@ skipping 3506 matching lines @@
         return;
 
     Vector<RefPtr<Element> > associatedFormControls;
     copyToVector(m_associatedFormControls, associatedFormControls);
 
     frame()->page()->chrome()->client()->didAssociateFormControls(associatedFormControls);
     m_associatedFormControls.clear();
 }
 
 } // namespace WebCore