Troubles with generating generic contract

[mishioo]

From the following C# code:

public class ValidateImportActivityTranslationsResult
{
   public ValidationResult<Guid> ActivityDoesNotExist { get; set; }
}

public class ValidationResult<TElement>
{
   public bool IsInvalid { get; set; }
   public List<TElement> Elements { get; set; }
}

an invalid dart class is generated:

@ContractsSerializable(genericArgumentFactories: true)
class ValidationResult<TElement> with EquatableMixin {
  ValidationResult({
    required this.isInvalid,
    required this.elements,
  });

  factory ValidationResult.fromJson(
    Map<String, dynamic> json,
    TElement Function(Object?) fromJsonTElement,
  ) =>
      _$ValidationResultFromJson(json, fromJsonTElement);

  final bool isInvalid;

  final List<TElement> elements;

  get props => [isInvalid, elements];
}

The toJson method could not have been generated, as per the error message coming from build_runner:

Could not generate `toJson` code for `activityDoesNotExist`.
To support the type `ValidationResult` you can:
* Use `JsonConverter`
  https://pub.dev/documentation/json_annotation/latest/json_annotation/JsonConverter-class.html
* Use `JsonKey` fields `fromJson` and `toJson`
  https://pub.dev/documentation/json_annotation/latest/json_annotation/JsonKey/fromJson.html
  https://pub.dev/documentation/json_annotation/latest/json_annotation/JsonKey/toJson.html
package:companion/data/contracts/contracts.dart:16864:34
      ╷
16864 │   final ValidationResult<String> activityDoesNotExist;
      │                                  ^^^^^^^^^^^^^^^^^^^^

Software/package versions used:

Flutter 3.29.2 • channel  • https://github.com/flutter/flutter.git
Framework • revision archlinuxa (unknown (arch linux aur package)) • 2038-01-19 03:14:08
Engine • revision 18b71d647a
Tools • Dart 3.7.0 • DevTools 2.42.2

leancode_contracts_generator: 0.9.0