check if webcontents are available at tabs_api

chrome/browser/extensions/api/tabs/tabs_api.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/extensions/api/tabs/tabs_api.h"
 
 #include <stddef.h>
 #include <algorithm>
 #include <limits>
 #include <memory>
@@ skipping 921 matching lines @@
     }
 
     TabStripModel* tab_strip = browser->tab_strip_model();
     for (int i = 0; i < tab_strip->count(); ++i) {
       WebContents* web_contents = tab_strip->GetWebContentsAt(i);
       memory::TabManager* tab_manager = g_browser_process->GetTabManager();
 
       if (index > -1 && i != index)
         continue;
 
+      if (!web_contents) {
+        continue;
+      }
+      
       if (!MatchesBool(params->query_info.highlighted.get(),
                        tab_strip->IsTabSelected(i))) {
         continue;
       }
 
       if (!MatchesBool(params->query_info.active.get(),
                        i == tab_strip->active_index())) {
         continue;
       }
 
@@ skipping 1193 matching lines @@
       params->tab_id
           ? ErrorUtils::FormatErrorMessage(keys::kCannotDiscardTab,
                                            base::IntToString(*params->tab_id))
           : keys::kCannotFindTabToDiscard));
 }
 
 TabsDiscardFunction::TabsDiscardFunction() {}
 TabsDiscardFunction::~TabsDiscardFunction() {}
 
 }  // namespace extensions