Problem generating mapper.g.dart

[dany2709]

I have been trying to generate the mapper.g.dart after configuring everything in the classes and I always get the same error.

flutter pub run build_runner build --delete-conflicting-outputs --verbose

[SEVERE] simple_json:simple_json on lib/$lib$:

FormatException: Unexpected extension byte (at offset 97)
dart:async                                                        _rootRunUnary
package:build_resolvers/src/build_asset_uri_resolver.dart 120:23  BuildAssetUriResolver._updateCachedAssetState
package:build_resolvers/src/build_asset_uri_resolver.dart 75:16   BuildAssetUriResolver.performResolve
package:build_resolvers/src/resolver.dart 128:11                  PerActionResolver._resolveIfNecessary.<fn>
package:pool/pool.dart 127:14                                     Pool.withResource
package:build_resolvers/src/resolver.dart 91:5                    PerActionResolver.isLibrary
package:simple_json/src/json_mapper_builder.dart 42:12            JsonMapperBuilder.build

SessionModel.dart

import 'package:project/core/di/di.dart';
import 'package:project/core/service/storage_service.dart';
import 'package:project/features/authentication/data/models/user_model.dart';
import 'package:project/features/authentication/domain/entities/session_entity.dart';
import 'package:json_annotation/json_annotation.dart';
import 'package:simple_json_mapper/simple_json_mapper.dart';

part 'session_model.g.dart';

@JsonObject()
@JsonSerializable()
class SessionModel extends Session {
  const SessionModel({
    required this.token,
    required this.tokenType,
    required this.realm,
    required this.user,
  }) : super(
          token: token,
          tokenType: tokenType,
          realm: realm,
          user: user,
        );

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

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

  // save instance on SecureStorage
  @override
  Future<void> save() async {
    return getIt<Storage>().write<Session>(key: Session.key, obj: this);
  }

  // @JsonProperty(name: 'access_token')

  @JsonProperty(name: 'access_token')
  @JsonKey(name: 'access_token')
  final String token;

  @JsonProperty(name: 'token_type')
  @JsonKey(name: 'token_type')
  final String tokenType;

  @JsonProperty(name: 'realm')
  @JsonKey(name: 'realm')
  final String realm;

  @JsonProperty(name: 'user')
  @JsonKey(name: 'user')
  final UserModel user;

  @override
  List<Object?> get props => [
        token,
        tokenType,
        realm,
        user,
      ];
}

I am using json_annotation to generate fromJson and toJson, I don't think that is what is generating the error. Any idea what I might be doing wrong?