我正在使用YouTube上的教程中的flutter练习这个covid-19追踪器应用程序，所以在编写了一些代码后，当我点击API并运行应用程序时，它抛出了这个异常，我不明白为什么，因为我是一个初学者，也是Flutter的新手。

import 'dart:convert';

import 'package:covid_tracker/Model/world_states_model.dart';
import 'package:http/http.dart' as http;

import 'Utilities/app_url.dart';

class StatesServices {
// async means waiting for your request

  Future<WorldStatesModel> fetchWorldStatesRecords() async {
    final response = await http.get(Uri.parse(AppUrl.worldStatesApi));

    if (response.statusCode == 200) {
      var data = jsonDecode(response.body);
      return WorldStatesModel.fromJson(data);
    } else {
      throw Exception('Error');
    }
  }
}

class AppUrl {
// this is our base url
  static const String baseUrl = 'https://disease.sh/v3/covid-19';

// fetch world covid states
  static const String worldStatesApi = '${baseUrl}all';
  static const String countriesList = '${baseUrl}countries';
}