CowState constructor Null safety

1. farmershands
2. cow_state
3. CowState
4. CowState constructor

CowState class

1. Constructors
2. CowState
3. Properties
4. cowGroupState
5. cows
6. cowStatusToday
7. cowStatusYesterday
8. groupCows
9. hashCode
10. hasListeners
11. runtimeType
12. selectedCows
13. Methods
14. addListener
15. createCow
16. deselectAll
17. dispose
18. init
19. noSuchMethod
20. notifyListeners
21. removeCows
22. removeCowsFromGroup
23. removeListener
24. toString
25. updateCows
26. Operators
27. operator ==