How to update a map with map data from API in Flutter

I am new to Flutter and finding it difficult to update _users with a map I get from API. I get build errors even though no actual errors are shown in VSCode.

Here is my code.

import 'dart:convert';

import 'package:flutter/material.dart';
import 'package:firebase_auth/firebase_auth.dart';
import 'package:http/http.dart' as http;

class Users with ChangeNotifier {
  final String id;
  final String firstName;
  final String lastName;
  final String emailAddress;
  final String phoneNumber;
  final String classId;
  final String roleId;
  final String dateCreated;
  final String uid;
  bool isActive;
  final String profilePhoto;

  Users({
    required this.id,
    required this.firstName,
    required this.lastName,
    required this.emailAddress,
    required this.phoneNumber,
    required this.classId,
    required this.roleId,
    required this.dateCreated,
    required this.uid,
    required this.isActive,
    required this.profilePhoto,
  });

  Map<String, Users> _users = {};

  Map<String, Users> get users {
    return {..._users};
  }

  Future<void> fetchUser() async {
    try {
      final String idToken =
          await FirebaseAuth.instance.currentUser!.getIdToken();

      final reauthUrl = Uri.parse(
          'https://example-backend.azurewebsites.net/api/v1/User/reauth');

      final reauthResponse = await http.post(reauthUrl,
          headers: {
            'Content-Type': 'application/json',
            'Accept': 'application/json'
          },
          body: json.encode({"token": idToken}));

      final returnedTokenData =
          json.decode(reauthResponse.body) as Map<String, dynamic>;

      final serverToken = returnedTokenData['token'];

      final url =
          Uri.parse('https://example-backend.azurewebsites.net/api/v1/User');

      final response = await http.get(
        url,
        headers: {'Authorization': 'Bearer $serverToken'},
      );

      final responseData = json.decode(response.body) as Map<String, dynamic>;
      Map<String, dynamic> userData = {};
      userData.update(id, (value) {
        return Users(
          id: responseData['id'],
          firstName: responseData['firstName'],
          lastName: responseData['lastName'],
          emailAddress: responseData['emailAddress'],
          phoneNumber: responseData['phoneNumber'],
          classId: responseData['classId'],
          roleId: responseData['roleId'],
          dateCreated: responseData['dateCreated'],
          uid: responseData['uid'],
          isActive: responseData['isActive'],
          profilePhoto: responseData['profilePhoto'],
        );
      });
      print(userData);

      notifyListeners();
    } catch (error) {
      rethrow;
    }
  }
}