Fix changing subject weights

[Jonas-Sander]

Fixes #1803

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[Jonas-Sander]

(Just a note for myself)

Inside the private grade logic a class called NonNegativeWeight is used.

class NonNegativeWeight extends Weight {
  factory NonNegativeWeight.fromWeight(Weight weight) =>
      NonNegativeWeight.factor(weight.asFactor);
  NonNegativeWeight.factor(num factor) : super.factor(factor) {
    if (factor < 0) {
      throw ArgumentError('Weight must be non-negative');
    }
  }
  NonNegativeWeight.percent(num percent) : super.percent(percent) {
    if (percent < 0) {
      throw ArgumentError('Weight must be non-negative');
    }
  }
}

Inside subject_settings_page_controller.dart we used the following code:

  late IMap<GradeTypeId, Weight> _weights;
  // ...
   SubjectSettingsPageController({
      required this.subRef,
      required this.gradesService,
    }) {
    //...
    _weights = _getTerm()!.gradeTypeWeightings;
    //...
  }

This threw the following error:

  type 'Weight' is not a subtype of type 'NonNegativeWeight' of 'value'
  package:fast_immutable_collections/src/imap/imap.dart 822:27                                       IMap.add
  package:sharezone/grades/pages/subject_settings_page/subject_settings_page_controller.dart 142:25  SubjectSettingsPageController.setGradeWeight

This is unexpected for the consuming code of the grades logic.
I need to create a fix so that the consuming code does not break.
Probably it should never see NonNegativeWeight - maybe make it a completely different class?

[nilsreichardt]

Thanks for fixing ❤️ I added the bug back then 😭