message: mission: set activity before calling callback
  And don't forget to set activity.
diff: @@ -117,9 +117,10 @@ void MissionImpl::process_mission_ack(const mavlink_message_t &message)
  _last_current_mavlink_mission_item = -1;
  _last_reached_mavlink_mission_item = -1;
  + _activity = Activity::NONE;
  +
  report_mission_result(_result_callback, Mission::Result::SUCCESS);
  LogInfo() << "Mission accepted";
  - _activity = Activity::NONE;
  } else if (mission_ack.type == MAV_MISSION_NO_SPACE) {
  LogErr() << "Error: too many waypoints: " << int(mission_ack.type);
  report_mission_result(_result_callback, Mission::Result::TOO_MANY_MISSION_ITEMS);
  @@ -194,6 +195,7 @@ void MissionImpl::upload_mission_async(const std::vector<std::shared_ptr<Mission
  report_mission_result(callback, Mission::Result::ERROR);
  }
  + _activity = Activity::SET_MISSION;
  _result_callback = callback;
  }
  