Populate data Map before opening screen

Question

I am trying to get data from an API to populate a Map and then display the screen to show the data from the Map

My getter is as follows:

Map data = {};
Future getData(symbol) async {
  final response = await http.get(Uri.parse(
      "https://financialmodelingprep.com/api/v3/quote/" +
          symbol +
          "?apikey=${api_key}"));
  if (response.statusCode == 200) {
    final jsonResponse = json.decode(response.body);
    // print(jsonResponse);
    data = jsonResponse[0];
    print(data);
  }
}

For starters, I am trying to display the symbol and name from the data in the API:

void initState() {
    getData(data['symbol']);
    super.initState();
  }

Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Colors.white,
      appBar: AppBar(
        toolbarHeight: 80,
        centerTitle: true,
        backgroundColor: Colors.white,
        automaticallyImplyLeading: true,
        title: Column(
          children: [
            Text("Symbol", style: TextStyle(color: Colors.black)),
            Text("Name", style: TextStyle(color: Colors.black))
          ],
        ),
      ),
    );
  }

After opening one screen successfully, on opening the next screen, I get the error:

type 'Null' is not a subtype of type 'String'

Can anyone suggest steps to deal with the issue? Any help would be appreciated.

Thanks