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');
    }
  }
}

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