cows property Null safety

1. farmershands
2. cow_current_status_screen_vm
3. CowCurrentStatusScreenVM
4. cows property

CowCurrentStatusScreenVM class

1. Constructors
2. CowCurrentStatusScreenVM
3. Properties
4. cowDatas
5. cows
6. cowStatusToday
7. cowStatusYesterday
8. dataTableColumns
9. dataTableRows
10. hashCode
11. hasListeners
12. refreshController
13. runtimeType
14. selectedDateTime
15. sortColumnIdx
16. sortIsAscending
17. Methods
18. addListener
19. checkIfCowGroupChanged
20. dispose
21. fetchCowData
22. fetchCowDataListener
23. fetchCowsCntByStatus
24. init
25. noSuchMethod
26. notifyListeners
27. onLoading
28. onPressCalendar
29. onPressNextDate
30. onPressPrevDate
31. onRefresh
32. onSelectChanged
33. removeListener
34. requestRefresh
35. toString
36. updateDataTableColumns
37. updateDataTableRows
38. Operators
39. operator ==