NULL check before calling thumbnail_db_ method.

chrome/browser/history/history_backend.cc

 // Copyright (c) 2012 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/history/history_backend.h"
 
 #include <algorithm>
 #include <functional>
 #include <list>
 #include <map>
@@ skipping 641 matching lines @@
   if (!archived_db_->Init(archived_name)) {
     LOG(WARNING) << "Could not initialize the archived database.";
     archived_db_.reset();
   }
 
   // Generate the history and thumbnail database metrics only after performing
   // any migration work.
   if (base::RandInt(1, 100) == 50) {
     // Only do this computation sometimes since it can be expensive.
     db_->ComputeDatabaseMetrics(history_name);
-    thumbnail_db_->ComputeDatabaseMetrics();
+    if (thumbnail_db_)
+      thumbnail_db_->ComputeDatabaseMetrics();
   }
 
   // Tell the expiration module about all the nice databases we made. This must
   // happen before db_->Init() is called since the callback ForceArchiveHistory
   // may need to expire stuff.
   //
   // *sigh*, this can all be cleaned up when that migration code is removed.
   // The main DB initialization should intuitively be first (not that it
   // actually matters) and the expirer should be set last.
   expirer_.SetDatabases(db_.get(), archived_db_.get(),
@@ skipping 2326 matching lines @@
   int rank = kPageVisitStatsMaxTopSites;
   std::map<GURL, int>::const_iterator it = most_visited_urls_map_.find(url);
   if (it != most_visited_urls_map_.end())
     rank = (*it).second;
   UMA_HISTOGRAM_ENUMERATION("History.TopSitesVisitsByRank",
                             rank, kPageVisitStatsMaxTopSites + 1);
 }
 #endif
 
 }  // namespace history