Null check operator used on a null value package:source_gen/src/type_checker.dart 215:74 TypeChecker.isSuperTypeOf

[romanlapa]

Describe the bug
I am getting this error when I try to generate a retrofit service.

Null check operator used on a null value
package:source_gen/src/type_checker.dart 215:74             TypeChecker.isSuperTypeOf

yaml:

name: data
description: "A new Flutter project."
version: 0.0.1
homepage:
publish_to: none

environment:
  sdk: ^3.5.1
  flutter: ">=1.17.0"

dependencies:
  flutter:
    sdk: flutter

  domain:
    path: ../domain

  shared_preferences: ^2.3.3
  dio: ^5.7.0
  retrofit: ^4.4.1
  logger: ^2.4.0  # for logging purpose
  json_annotation: ^4.9.0

dev_dependencies:
  flutter_test:
    sdk: flutter
  flutter_lints: ^4.0.0
  build_runner: ^2.4.13
  retrofit_generator: '>=8.0.0 <10.0.0' # for Dart 3.3 use ^9.0.0
  
global_options:
  freezed:
    runs_before:
      - json_serializable
  json_serializable:
    runs_before:
      - retrofit_generator

model:

class Token {

	Token(this.token);

	String? token;

	factory Token.fromJson(Map<String, String?> data) {
		if (data
		case {
		'token': String token,
		}) {
			return Token(token);
		} else {
			if (data['token'] is! String) {
				throw FormatException('Invalid JSON: required "token" field of type String in $data');
			}
			throw FormatException('Invalid JSON: $data');
		}
	}

	Map<String, String?> toJson() {
		// return a map literal with all the non-null key-value pairs
		return {
			'token': token,
		};
	}

}
Or like this:
import 'package:json_annotation/json_annotation.dart';

part 'token.g.dart';

@JsonSerializable()
class Token {

	Token(this.token);

	final String? token;

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

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

}

service:

import 'package:data/models/Token.dart';
import 'package:dio/dio.dart';
import 'package:retrofit/retrofit.dart';

part '***.g.dart';

@RestApi(baseUrl: "https://dev-api.***.com")
abstract class AppRetrofitService {
  // helper methods that help you instanciate your service
  factory AppRetrofitService(Dio dio, {String baseUrl}) = _AppRetrofitService;

  @POST("/api/v1/login")
  Future<Token> login(@Body() Map<String, String> body);
  
}

To Reproduce
Steps to reproduce the behavior:
Run fvm dart run build_runner build --delete-conflicting-outputs or dart run build_runner build --delete-conflicting-outputs depends on do you use fvm or not.

Expected behavior
Successfuly generated class.

Additional context
Add any other context about the problem here.