Null check StyleEngine in ~ShadowRoot.

sky/engine/core/dom/shadow/ShadowRoot.cpp

 /*
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Neither the name of Google Inc. nor the names of its
@@ skipping 38 matching lines @@
 ShadowRoot::ShadowRoot(Document& document)
     : DocumentFragment(0, CreateShadowRoot)
     , TreeScope(*this, document)
     , m_registeredWithParentShadowRoot(false)
     , m_descendantInsertionPointsIsValid(false)
 {
 }
 
 ShadowRoot::~ShadowRoot()
 {
-    document().styleEngine()->didRemoveShadowRoot(this);
+    if (StyleEngine* styleEngine = document().styleEngine())
+        styleEngine->didRemoveShadowRoot(this);
 
     // We cannot let ContainerNode destructor call willBeDeletedFromDocument()
     // for this ShadowRoot instance because TreeScope destructor
     // clears Node::m_treeScope thus ContainerNode is no longer able
     // to access it Document reference after that.
     willBeDeletedFromDocument();
 
     // We must remove all of our children first before the TreeScope destructor
     // runs so we don't go through TreeScopeAdopter for each child with a
     // destructed tree scope in each descendant.
@@ skipping 143 matching lines @@
     Vector<RefPtr<InsertionPoint> > insertionPoints;
     for (InsertionPoint* insertionPoint = Traversal<InsertionPoint>::firstWithin(*this); insertionPoint; insertionPoint = Traversal<InsertionPoint>::next(*insertionPoint, this))
         insertionPoints.append(insertionPoint);
 
     ensureShadowRootRareData()->setDescendantInsertionPoints(insertionPoints);
 
     return m_shadowRootRareData->descendantInsertionPoints();
 }
 
 }