fetchGraphData method Null safety

Implementation

Future<List<BarDataDay>?> fetchGraphData() async {
  currentStartAt = selectedDateTime.subtract(Duration(days: _selectedGraphViewType.subDays));
  currentEndAt = selectedDateTime;
  List<CowData>? cowData = await ApiService().fetchCowData(queryParameters: {
    "cow_ids": '["${report.cow?.id}"]',
    "start_at": DateFormat(StringConstants.timeYMDDashFormat).format(selectedDateTime.subtract(Duration(days: _selectedGraphViewType.subDays))),
    "end_at": DateFormat(StringConstants.timeYMDDashFormat).format(selectedDateTime),
    "hour_unit": _selectedGraphViewType.hourUnit.toString(),
  });
  barItemsDay = Helper().convertCowDataToGraphData(
    cowData: cowData,
    targetDateTime: selectedDateTime,
    continuedDays: report.continuedDays == null ? 0 : report.continuedDays!,
    graphViewType: _selectedGraphViewType,
    actionType: selectedAction,
  );
  return barItemsDay;
}