fetchCowsCntByStatus method Null safety

Implementation

Future<void> fetchCowsCntByStatus() async {
  cowStatusYesterday = null;
  cowStatusToday = null;
  cowStatusByDayAfterBirth1_15 = null;
  cowStatusByDayAfterBirth16_30 = null;
  cowStatusByDayAfterBirth31_60 = null;
  cowStatusByDayAfterBirth61_99999 = null;
  notifyListeners();

  cowStatusYesterday = await ApiService().fetchCowsCntByStatus(targetAt: DateTime.now().subtract(Duration(days: 2)), dayAfterBirthStart: 1, dayAfterBirthEnd: 99999);
  cowStatusToday = await ApiService().fetchCowsCntByStatus(targetAt: DateTime.now().subtract(Duration(days: 1)), dayAfterBirthStart: 1, dayAfterBirthEnd: 99999);
  cowStatusByDayAfterBirth1_15 = await ApiService().fetchCowsCntByStatus(targetAt: DateTime.now().subtract(Duration(days: 1)), dayAfterBirthStart: 1, dayAfterBirthEnd: 15);
  cowStatusByDayAfterBirth16_30 = await ApiService().fetchCowsCntByStatus(targetAt: DateTime.now().subtract(Duration(days: 1)), dayAfterBirthStart: 16, dayAfterBirthEnd: 30);
  cowStatusByDayAfterBirth31_60 = await ApiService().fetchCowsCntByStatus(targetAt: DateTime.now().subtract(Duration(days: 1)), dayAfterBirthStart: 31, dayAfterBirthEnd: 60);
  cowStatusByDayAfterBirth61_99999 = await ApiService().fetchCowsCntByStatus(targetAt: DateTime.now().subtract(Duration(days: 1)), dayAfterBirthStart: 60, dayAfterBirthEnd: 99999);
  notifyListeners();
}