sync: fix tiny bug with NotificationServiceSessionsRouter

chrome/browser/sync/sessions2/notification_service_sessions_router.cc

 // Copyright 2013 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 "chrome/browser/sync/sessions2/notification_service_sessions_router.h"
 
 #include "base/logging.h"
 #include "chrome/browser/chrome_notification_types.h"
 #include "chrome/browser/extensions/tab_helper.h"
 #include "chrome/browser/favicon/favicon_changed_details.h"
@@ skipping 117 matching lines @@
   if (!flare_.is_null()) {
     flare_.Run(syncer::SESSIONS);
     flare_.Reset();
   }
 }
 
 void NotificationServiceSessionsRouter::OnNavigationBlocked(
     content::WebContents* web_contents) {
   SyncedTabDelegate* tab =
       SyncedTabDelegate::ImplFromWebContents(web_contents);
-  if (!tab)
+  if (!tab || !handler_)
     return;
 
   DCHECK(tab->profile() == profile_);
   handler_->OnLocalTabModified(tab);
 }
 
 void NotificationServiceSessionsRouter::StartRoutingTo(
     LocalSessionEventHandler* handler) {
   DCHECK(!handler_);
   handler_ = handler;
 }
 
 void NotificationServiceSessionsRouter::Stop() {
   weak_ptr_factory_.InvalidateWeakPtrs();
   registrar_.RemoveAll();
   handler_ = NULL;
 }
 
 }  // namespace browser_sync