def UnregisterMessageHandler(self, timeout=None):
    """Unregisters any registered message handler."""
    if self.handler_thread:
      self.handler_stop = True
      self.handler_thread.join(timeout)
      if self.handler_thread.isAlive():
        raise RuntimeError("Message handler thread did not join in time.")
      self.handler_thread = None
Unregisters any registered message handler.