Flutter Unhandled Exception: type 'Null' is not a subtype of type 'Response' in type cast

Question

I can't seem to figure out the issue in my flutter GetConnect request, I am using the Getx's Getconnect library to send an API request to authenticate users, the requests are authenticated successfully but then the app throws an error when I try to get the user data and token back,

class AuthService {
  final IHttpConnect _connect;

  const AuthService(IHttpConnect connect) : _connect = connect;

  String get _prefix => 'auth';

  Future<AuthResponse> authenticateUser(
    var body,
  ) async {
    final response = await _connect.post(
      '$_prefix/login',
      body,
      decoder: (value){
        print(value);
        AuthResponse data = AuthResponse.fromJson(
          value as Map<String, dynamic>,
        );

        return data;
      },
    );
    
    if (response.success) {
      return response.payload!;
    } else {
      switch (response.statusCode) {
        case 404:
          throw UserNotFoundException();
        default:
          throw DefaultException(message: response.payload!.error!);
      }
    }
  }
}

My AuthResponse model looks like this

import 'package:json_annotation/json_annotation.dart';

part 'auth_response.g.dart';


@JsonSerializable()
class AuthResponse {
  AuthResponse({
    required this.success,
     this.data,
     this.error,
  });

  final bool success;
  final Data? data;
  final String? error;

  factory AuthResponse.fromJson(Map<String, dynamic> json) => _$AuthResponseFromJson(json);

  Map<String, dynamic> toJson() => _$AuthResponseToJson(this);
}

@JsonSerializable()
class Data {
  Data({
    required this.token,
    required this.user,
  });

  final String token;
  final User user;

  factory Data.fromJson(Map<String, dynamic> json) => _$DataFromJson(json);

  Map<String, dynamic> toJson() => _$DataToJson(this);
}

@JsonSerializable()
class User {
  User({
    required this.id,
    required this.name,
    required this.email,
    required this.emailVerifiedAt,
    required this.createdAt,
    required this.updatedAt,
  });


  final int id;
  final String name;
  final String email;
  @JsonKey(name: 'email_verified_at')
  final DateTime? emailVerifiedAt;
  @JsonKey(name: 'created_at')
  final DateTime createdAt;
  @JsonKey(name: 'updated_at')
  final DateTime updatedAt;

  factory User.fromJson(Map<String, dynamic> json) => _$UserFromJson(json);

  Map<String, dynamic> toJson() => _$UserToJson(this);
}

If I print the data I get

{success: true, data: {token: 157|4YXMdrMwDIECxsVSf5hIeON5scCu9lZTQP2B5wXa, user: {id: 1, name: Kenya Friesen, email: mdavis@yahoo.com, email_verified_at: 2022-11-18T00:27:42.000000Z, created_at: 2022-11-18T00:27:42.000000Z, updated_at: 2022-11-18T00:27:42.000000Z}}, error: ddd}

IHttpConnect class

import './response.model.dart';

abstract class IHttpConnect {
  Future<Response<T>> get<T>(
    String url, {
    required T Function(dynamic)? decoder,
  });

  Future<Response<T>> post<T>(
    String url,
    Map<String, dynamic> body, {
    T Function(dynamic)? decoder,
  });

  Future<Response<T>> put<T>(
    String url,
    Map<String, dynamic> body, {
    T Function(dynamic)? decoder,
  });

  Future<Response<T>> patch<T>(
    String url,
    Map<String, dynamic> body, {
    T Function(dynamic)? decoder,
  });

  Future<Response<T>> delete<T>(
    String url, {
    required T Function(dynamic)? decoder,
  });
}

Response Class

class Response<T> {
  final int statusCode;
  final T? payload;

  bool get success => statusCode <= 200;
  
  const Response({
    required this.statusCode,
    required this.payload,
  });
}

I appreciate any assistance