static void CloseScreen()
 {
     if (master != null)
     {
         if (master.InvokeRequired)
         {
             MethodInvoker InvalidateMe = delegate
             {
                 master.Close();
             };
             try
             {
                 master.Invoke(InvalidateMe);
             }
             catch
             {
             }
         }
         else
         {
             master.Close();
             master = null;
         }
     }
 }