Uncaught TypeError: Object prototype may only be an Object or null: undefined on promise

I am making some mock up for the company I work to, I am making an API call to send an XML file but the client throws Uncaught TypeError: Object prototype may only be an Object or null: undefined just before sending the request. I read that the imports are the problem but I can’t figure it out what, even because my company bought a very crappy tool to auto generate the server and client codes, the models, the api functions ecc ecc… but it sucks.

This is the call where the client fails:

upload() {
    console.log('mando richiesta 1');
    const fattura = this.delegates.parser.PostFile(this.file).toPromise().then((value) => {console.log(value)});
   console.log(fattura);
}

the this.delegates.parser.PostFile(x) is a function generated by the tool I mentioned before and I didn’t wrote it. It contains:

public PostFile(xmlFile: any) {
    return this.request<IPostFileResponse>('POST', 'PostFile', { xmlFile }, false);
}

the tool-generated request<T>(x) function:

protected request<TResponse>(method: 'DELETE' | 'GET' | 'HEAD' | 'JSONP' | 'OPTIONS' | 'POST' | 'PUT' | 'PATCH', resource: string, payload: any, disableContextAttach = false) {
    const url = this.api + resource;
    const options: IShHttpRequestBodyOptions<TResponse> = {};
    const serialized = this.serializer.serialize(payload);
    if (method.startsWith('P')) {
      options.headers = { 'Content-Type': 'application/json' };
      options.body = serialized;
    } else {
      if (typeof payload === 'object') {
        for (const key in payload) {
          const element = payload[key];
          if (isNoU(element)) {
            delete payload[key];
          }
        }
      }
      options.params = payload;
    }
    if (this.activity && this.activity.taskId && method !== 'JSONP') {
      options.headers = {
        'Content-Type': 'application/json',
        'Activity-TaskId': this.activity.taskId
      };
    }
    if (options) {
      options.observe = 'response' as 'body';
      options.responseType = 'text' as 'json';
    }
    let request: Observable<TResponse> = this.http.request<TResponse, any>(method, url, options)
      .pipe(map(x => this.deserialize.call(this, x)));
    if (!disableContextAttach) {
      if (payload) {
        this.attach(payload);
      }
      request = request.pipe(map(this.attach.bind(this)));
    }
    return request;
  }

and server side I have:

[HttpPost("postFile")]
// <custom:customAttributesPostFile>
// </custom:customAttributesPostFile>
public async Task<ActionResult<PostFileResponse>> PostFile([FromForm] PostFileRequest request, CancellationToken requestAborted)
{
  // <custom:PostFile>
  XmlSerializer ser = new XmlSerializer(typeof(FatturaElettronicaTypeDTO));
  Stream stream = (request.XmlFile as IFormFile).OpenReadStream();
  FatturaElettronicaTypeDTO deserialized = (FatturaElettronicaTypeDTO)ser.Deserialize(stream);

  _fatturaSelezionata.fatturaSelez = deserialized;

  var fatturaDto = _fatturaMapper.mapFattura(deserialized);

  return new PostFileResponse() { Fattura = fatturaDto }; /*fatturaDto*/
  // </custom:PostFile>
}