Check for the default search engine being null before using.

components/search_engines/template_url_service.h

 // 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.
 
 #ifndef COMPONENTS_SEARCH_ENGINES_TEMPLATE_URL_SERVICE_H_
 #define COMPONENTS_SEARCH_ENGINES_TEMPLATE_URL_SERVICE_H_
 
 #include <stddef.h>
 
 #include <list>
@@ skipping 239 matching lines @@
   bool CanMakeDefault(const TemplateURL* url) const;
 
   // Set the default search provider.  |url| may be null.
   // This will assert if the default search is managed; the UI should not be
   // invoking this method in that situation.
   void SetUserSelectedDefaultSearchProvider(TemplateURL* url);
 
   // Returns the default search provider. If the TemplateURLService hasn't been
   // loaded, the default search provider is pulled from preferences.
   //
-  // NOTE: At least in unittest mode, this may return NULL.
+  // NOTE: This may return null in certain circumstances such as:
+  //       1.) Unit test mode
+  //       2.) The default search engine is disabled by policy.
   const TemplateURL* GetDefaultSearchProvider() const;
 
   // Returns true if the |url| is a search results page from the default search
   // provider.
   bool IsSearchResultsPageFromDefaultSearchProvider(const GURL& url) const;
 
   // Returns true if the default search is managed through group policy.
   bool is_default_search_managed() const {
     return default_search_provider_source_ == DefaultSearchManager::FROM_POLICY;
   }
@@ skipping 574 matching lines @@
   // Helper class to manage the default search engine.
   DefaultSearchManager default_search_manager_;
 
   std::unique_ptr<GoogleURLTracker::Subscription>
       google_url_updated_subscription_;
 
   DISALLOW_COPY_AND_ASSIGN(TemplateURLService);
 };
 
 #endif  // COMPONENTS_SEARCH_ENGINES_TEMPLATE_URL_SERVICE_H_