`Param.selectComponent` throwing `io.mavsdk.param.Param$ParamException: UNKNOWN: Unknown`

[benc-riis]

I'm working on a project where I'll be sending some custom data over MAVLink to a companion computer on my drone. I wanted to use PARAM_EXT_SET to prototype my setup before looking into creating a custom message since I'm just wanting to send a short string. But I've run into an issue trying to use the param plugin. Here's a snippet:

Param param = system.getParam();
Disposable d = param.selectComponent(191, Param.ProtocolVersion.EXT).subscribe(
        () -> {
            Disposable d2 = param.setParamCustom("MY_PARAM", "MY_VALUE").subscribe(
                    () -> Timber.d("Updated parameter"),
                    e -> Timber.e(e, "Failed to update parameter")
            );
        },
        e -> Timber.e(e, "Error selecting companion computer for setting parameters")
);

This code runs into an error in the param.selectComponent call:

Error selecting companion computer for setting parameters
io.mavsdk.param.Param$ParamException: UNKNOWN: Unknown
...

Changing the component ID to the autopilot (1) and using ProtocolVersion.V1 does not solve the issue. I can set parameters on my autopilot if I remove the selectComponent call since it's the default component selected. I can also send the PARAM_EXT_SET message fine with MAVLink direct to accomplish my goal, but then I would have to manually deal with resending the message if I don't receive an ACK.

Looking at the MAVSDK source for the plugin, I can see that ParamImpl::select_component always returns Param::Result::Unknown. I haven't had time to try messing around with and building MAVSDK myself, but could the fix be as simple as changing this return value to a success?

[julianoes]

Hm, what's the mavsdk version? And can you run the mavsdk_server binary separate or at least get all its debug output to see what is going on. @JonasVautherin probably can help with setting that up, or point you to the docs.

[JonasVautherin]

When you create a System you can set the ip:port of mavsdk_server:

  /**
   * Create a System object.The plugins are initialized lazily, when the corresponding
   * get method is called.
   *
   * @param host the address of mavsdk_server
   * @param port the port of mavsdk_server
   */
  public System(@NonNull String host, int port) {