message: core: make sure message handler is ready when used
diff: @@ -259,6 +259,9 @@ private:
  std::mutex _component_discovered_callback_mutex{};
  discover_callback_t _component_discovered_callback{nullptr};
  + // Needs to be before anything else because they can depend on it.
  + MAVLinkMessageHandler _message_handler{};
  +
  std::atomic<uint8_t> _system_id;
  uint64_t _uuid{0};
  @@ -290,11 +293,8 @@ private:
  static constexpr double _HEARTBEAT_SEND_INTERVAL_S = 1.0;
  MAVLinkParameters _params;
  -
  MAVLinkCommands _commands;
  - MAVLinkMessageHandler _message_handler{};
  -
  Timesync _timesync;
  TimeoutHandler _timeout_handler;
  