fetchCowsCntByStatus method Null safety

Implementation

Future<void> fetchCowsCntByStatus() async {
  cowStatusYesterday = null;
  cowStatusToday = null;
  notifyListeners();
  cowStatusYesterday = await ApiService().fetchCowsCntByStatus(targetAt: selectedDateTime.subtract(Duration(days: 1)), dayAfterBirthStart: 1, dayAfterBirthEnd: 99999, joinCowData: true);
  cowStatusToday = await ApiService().fetchCowsCntByStatus(targetAt: selectedDateTime, dayAfterBirthStart: 1, dayAfterBirthEnd: 99999, joinCowData: true);
  notifyListeners();
}