message: action_impl: only call existing callback
diff: @@ -413,8 +413,10 @@ void ActionImpl::command_result_callback(DeviceImpl::CommandResult command_resul
  {
  Action::Result action_result = action_result_from_command_result(command_result);
  + if (callback) {
  callback(action_result);
  }
  +}
  } // namespace dronelink
  