Issue 1694 - Fixed error when opening multiple tabs on Safari

safari/ext/background.js

 /*
  * This file is part of Adblock Plus <http://adblockplus.org/>,
  * Copyright (C) 2006-2014 Eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 42 matching lines @@
     },
     sendMessage: function(message, responseCallback)
     {
       this._messageProxy.sendMessage(message, responseCallback, {pageId: this._id});
     }
   };
 
   var isPageActive = function(page)
   {
     var tab = page._tab;
-    return tab == tab.browserWindow.activeTab && page == tab._visiblePage;
+    var win = tab.browserWindow;
+    return win && tab == win.activeTab && page == tab._visiblePage;
   };
 
   var forgetPage = function(id)
   {
     ext._removeFromAllPageMaps(id);
 
     delete pages[id]._tab._pages[id];
     delete pages[id];
   };
 
@@ skipping 596 matching lines @@
         page.activate();
         if (callback)
           callback(page);
         return;
       }
     }
 
     ext.pages.open(optionsUrl, callback);
   };
 })();