public static PopularFriendsWindow GetInstance(String accessToken) 
 {  
     if (PopularFriendsWindow.instance == null) 
     { 
         lock (_syncRoot) 
         { 
             if (PopularFriendsWindow.instance == null) 
             { 
                 PopularFriendsWindow.instance = new PopularFriendsWindow(accessToken); 
             } 
         } 
     }  
     return PopularFriendsWindow.instance; 
 }
 private void Window_Closed(object sender, EventArgs e)
 {
     instance = null;
 }