String text;

fetchData() async {
//...
  text = weatherData.weather[0].main ?? 'Waiting api response...';
//...
}

// in your build method
@override
Widget build(BuildContext context) {
  return Scaffold(
    body: Container(
      child: Text(text), //this will render "Waiting api response" first, and when the api result arrive, it will change
    ),
  );
}