Add null checks in navigator.serviceWorker access to fix possible crash

Source/modules/serviceworkers/NavigatorServiceWorker.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "config.h"
 #include "modules/serviceworkers/NavigatorServiceWorker.h"
 
 #include "core/frame/Navigator.h"
 #include "modules/serviceworkers/ServiceWorkerContainer.h"
 
@@ skipping 35 matching lines @@
 
 ServiceWorkerContainer* NavigatorServiceWorker::serviceWorker(ExecutionContext* executionContext)
 {
     if (!m_serviceWorker && frame())
         m_serviceWorker = ServiceWorkerContainer::create(executionContext);
     return m_serviceWorker.get();
 }
 
 void NavigatorServiceWorker::willDetachGlobalObjectFromFrame()
 {
-    m_serviceWorker->detachClient();
-    m_serviceWorker = nullptr;
+    if (m_serviceWorker) {
+        m_serviceWorker->detachClient();
+        m_serviceWorker = nullptr;
+    }
 }
 
 } // namespace WebCore