Don't show desktop notifications in Opera

background.js

 /*
  * This file is part of Adblock Plus <http://adblockplus.org/>,
  * Copyright (C) 2006-2013 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 568 matching lines @@
     {
       tabs.forEach(refreshIconAndContextMenu);
     });
   });
 }
 
 function showNotification(notification)
 {
   activeNotification = notification;
 
-  if (activeNotification.severity === "critical")
+  if (activeNotification.severity === "critical"
+      && typeof webkitNotifications !== "undefined")
   {
     var notification = webkitNotifications.createHTMLNotification("notification.html");
     notification.show();
     notification.addEventListener("close", prepareNotificationIconAndPopup);
   }
   else
     prepareNotificationIconAndPopup();
 }
 
 /**
@@ skipping 147 matching lines @@
     tabs.forEach(refreshIconAndContextMenu);
   });
 });
 
 setTimeout(function()
 {
   var notificationToShow = Notification.getNextToShow();
   if (notificationToShow)
     showNotification(notificationToShow);
 }, 3 * 60 * 1000);