docs: def RegisterFlowProcessingHandler(self, handler):
      """Registers a handler to receive flow processing messages."""
      self.UnregisterFlowProcessingHandler()
  
      if handler:
        self.flow_processing_request_handler_stop = False
        self.flow_processing_request_handler_thread = threading.Thread(
            name="flow_processing_request_handler",
            target=self._FlowProcessingRequestHandlerLoop,
            args=(handler,))
        self.flow_processing_request_handler_thread.daemon = True
        self.flow_processing_request_handler_thread.start()