Fix a crash on closing tab while the plugin settings bubble is visible

chrome/browser/ui/content_settings/content_setting_bubble_model.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/ui/content_settings/content_setting_bubble_model.h"
 
 #include "base/command_line.h"
 #include "base/prefs/pref_service.h"
 #include "base/strings/utf_string_conversions.h"
 #include "chrome/browser/chrome_notification_types.h"
@@ skipping 479 matching lines @@
 ContentSettingPluginBubbleModel::~ContentSettingPluginBubbleModel() {
   if (settings_changed()) {
     // If the user elected to allow all plugins then run plugins at this time.
     if (selected_item() == kAllowButtonIndex)
       OnCustomLinkClicked();
   }
 }
 
 void ContentSettingPluginBubbleModel::OnCustomLinkClicked() {
   content::RecordAction(UserMetricsAction("ClickToPlay_LoadAll_Bubble"));
-  DCHECK(web_contents());
+  // Web contents can be NULL if the tab was closed while the plugins
+  // settings bubble is visible.
+  if (!web_contents())
+    return;
 #if defined(ENABLE_PLUGINS)
   // TODO(bauerb): We should send the identifiers of blocked plug-ins here.
   ChromePluginServiceFilter::GetInstance()->AuthorizeAllPlugins(
       web_contents(), true, std::string());
 #endif
   set_custom_link_enabled(false);
   TabSpecificContentSettings::FromWebContents(web_contents())->
       set_load_plugins_link_enabled(false);
 }
 
@@ skipping 783 matching lines @@
   if (type == content::NOTIFICATION_WEB_CONTENTS_DESTROYED) {
     DCHECK_EQ(web_contents_,
               content::Source<WebContents>(source).ptr());
     web_contents_ = NULL;
   } else {
     DCHECK_EQ(chrome::NOTIFICATION_PROFILE_DESTROYED, type);
     DCHECK_EQ(profile_, content::Source<Profile>(source).ptr());
     profile_ = NULL;
   }
 }