cowData property Null safety

1. farmershands
2. cow_detail_screen_vm
3. CowDetailScreenVM
4. cowData property

CowDetailScreenVM class

1. Constructors
2. CowDetailScreenVM
3. Properties
4. barItemsDay
5. birth
6. cow
7. cowData
8. currentEndAt
9. currentStartAt
10. forceRender
11. gender
12. group
13. hashCode
14. hideGraph
15. isLoading
16. memo
17. prevSelectedGraphViewType
18. runtimeType
19. selectedAction
20. selectedDateTime
21. selectedGraphAvgType
22. selectedGraphViewType
23. title
24. todayCowData
25. todayValues
26. weight
27. yesterdayCowData
28. yesterdayValues
29. Methods
30. fetchAll
31. fetchGraphData
32. fetchTodayYesterdayData
33. init
34. loadMoreGraphData
35. noSuchMethod
36. onPressCalendar
37. onPressNextDate
38. onPressPrevDate
39. release
40. toString
41. Operators
42. operator ==