Pass a callback from Unity to JavaScript and back to Unity

I am trying to run an Action that sends data to the javascript on the browser, and when the browser finishes it runs a callback, and when the callback completes, it runs the item in the C# which runs the original callback.

Here are is the call order: GameSmart.User.IsGuest(Action origCallback) -> IsGuestUser(/*Executes the javascript*/) -> GuestResponse(string json) -> origCallback()

Once I compile and run the game, I get the following message in the chrome console:

MarshalDirectiveException: Cannot marshal type 'GameSmart.Response`1<GameSmart.IsGuestResponse>'.

I am not sure what that is saying or even means. Is there something I am doing wrong?

This is the class GameSmart.User:

public class User : API {
  [DllImport("__Internal")]
  public static extern void IsGuestUser(Response<IsGuestResponse> response);

  [MonoPInvokeCallback(typeof(Action))]
  public static void GuestResponse(Response<IsGuestResponse> r, string data) {
    r.callback(JsonUtility.FromJson<IsGuestResponse>(data));
  }

  public static void IsGuest(Action<IsGuestResponse> callback) {
    IsGuestUser(new Response<IsGuestResponse>(callback));
  }
}

Here are the Response/Respond classes:

public class Respond { }

public class Response<T> : Respond {
  public Action<T> callback;

  public Response(Action<T> cb) {
    callback = cb;
  }

  public void Action(Action act, params object[] args) {
    act();
  }

}

The JavaScript portion looks like this:

var GameSmartJs = {
  $GameSmartJs: {},

  IsGuestUser: function (obj) {
    gamesmart.user.isGuest(function (result) {
      this.runCallback('GuestResponse', obj, result);
    });
  },

  runCallback: function (callbackName, callback, result) {
    GameSmartJs[callbackName] = callback;
    Runtime.dynCall('vZ', callback, Pointer_stringify(result));
  }

};

autoAddDeps(GameSmartJs, '$GameSmartJs');
mergeInto(LibraryManager.library, GameSmartJs);

Edit

As per @Programmer suggested, to use _malloc and writeStringToMemory I tried this, and it produces the same error message.

IsGuestUser: function (obj) {
  gamesmart.user.isGuest(function (result) {
    GameSmartUser.GuestResponse = obj;
    var buffer = _malloc(lengthBytesUTF8(result) + 1);
    writeStringToMemory(result, buffer);
    Runtime.dynCall('vZ', obj, buffer);
  });
},