initDio method Null safety

Implementation

@override
void initDio() {
  super.initDio();
  SMHRequestRetrier requestRetrier = SMHRequestRetrier(Dio(dio.options));
  SMHRetryInterceptor retryInterceptor = SMHRetryInterceptor(requestRetrier);
  dio.interceptors.add(retryInterceptor);
  dio.options.followRedirects = false;
  dio.options.validateStatus = (status) {
    return status! < 400;
  };
}