Erro para fazer o chatbot

Olá. estou tendo esse erro:

poderiam me audar. segue abaixo os códigos.

import 'package:banco/screens/account_screen.dart';

void main(){
    AccountScreen accountScreen = AccountScreen();
    accountScreen.inicializeStream();
    accountScreen.runChatbot();
} 

import 'dart:async';
import 'package:banco/apikey.dart';
import 'package:banco/models/account.dart';
import 'package:http/http.dart';
import 'dart:convert';
class AccountService{
  final StreamController<String> _streamController = StreamController<String>();
  Stream<String> get streamInfos => _streamController.stream;
  String url = "https://api.github.com/gists/35f64ffee91c7789fd08184d21deab0a";


  Future<List<Account>> getAll() async{
    
    Response response = await get(Uri.parse(url));
    _streamController.add("${DateTime.now()} | requisicao de leitura");
    Map<String,dynamic> mapResponse = json.decode(response.body);
    List<dynamic> listDynamic = json.decode(mapResponse["files"]["accounts.json"]["content"]);
    List<Account> listAccounts = [];
    for (dynamic dyn in listDynamic){
      Map<String,dynamic> mapAccount = dyn as Map<String,dynamic>;
      Account account = Account.fromMap(mapAccount);
      listAccounts.add(account);
    }
    return listAccounts;
  }

  addAccount(Account account) async{
    List <Account> listAccounts = await getAll();
    listAccounts.add(account);
    List<Map<String,dynamic>> listContent=[];

    for (Account account in listAccounts){
      listContent.add(account.toMap());
    }
  String content = jsonEncode(listContent);
  Response response = await post(Uri.parse(url),headers: {
    "Authorization" : "Bearer $apiKey"
    }, body: json.encode({
      "description" : "accounts.json",
      "public":true,
      "files":{
        "accounts.json":{
          "content":content,
        }
      }

    }));
    if (response.statusCode.toString()[0]== '2'){
        _streamController.add("${DateTime.now()} | requisicao de adição bem sucedida(${account.name}).");

    }else{
        _streamController.add("${DateTime.now()} | requisicao de adição falhou(${account.name}).");

    }
  }
}

import 'dart:io';

import 'package:banco/models/account.dart';
import 'package:banco/services/accountservice.dart';

class AccountScreen {
  final AccountService _accountService = AccountService();

  void inicializeStream(){
    _accountService.streamInfos.listen((event) {
      print(event);
    },);
  }
  void runChatbot() async{
    print("Seja bem vindo ao Banco. Sou Roger.\n");

    bool isRunning = true;
    while(isRunning){
      print("Como posso te ajudar");
      print("Digite 1 para ver todas as contas");
      print("Digite 2 para adicionar uma conta");
      print("Digite 3 para sair.\n");
      String? input = stdin.readLineSync();
      if (input != null){
        switch(input){
          case '1':
          {
            await _getAllAccounts();
            break;
          }
          case '2':{
            await _addExampleAccount();
            break;
          }
          case '3':
          {
            isRunning = false;
            print("Te vejo na Proxima");
            break;
          }
          default:
          {
            print("Não entendi. Digite novamente");
          }
        }
      }
    }
  }

  _getAllAccounts()async{
    List<Account> listAccounts = await _accountService.getAll();
    print(listAccounts);
  }

  _addExampleAccount()async{
    Account example = Account(id: "ID555", name: "Jose", lastName: "Buzo", balance: 0);
    await _accountService.addAccount(example);
  }
  

}

class Account {
  String id;
  String name;
  String lastName;
  double balance;

  Account({required this.id,
    required this.name,
    required this.lastName,
    required this.balance});

  factory Account.fromMap(Map<String,dynamic>map){
    return Account(id: map["id"], name: map["name"], lastName: map["lastName"], balance: map["balance"],);
  }

  Map<String,dynamic> toMap(){
    return<String,dynamic>{
      "id" : id,
      "name" : name,
      "lastName" : lastName,
      "balance" : balance,
    };
  }
}