Инициализация поля

main.dart

void main() {
  
  final bar = Bar();

  print(bar.model.value);
  bar.model.value = "test";
  print(bar.model.value);
  bar.model = SomeModel('newModel');
  print(bar.model.value);

}

abstract class Model<T> {
  abstract T? value;
}

class SomeModel extends Model<String> {
  SomeModel(String param) {
    print('SomeModel');
    value = param;
  }

  @override
  String? value = 'default';
}

abstract class Foo<T> {
  abstract  Model<T> model;
}

class Bar extends Foo<String> {
  @override
  Model<String> model = SomeModel('initModel');
}