cowGroupState property Null safety

1. farmershands
2. api_service
3. ApiService
4. cowGroupState property

ApiService class

1. Constructors
2. ApiService
3. Properties
4. chos19
5. context
6. cowGroupState
7. cowState
8. demographic
9. deviceId
10. farmState
11. gatewayState
12. hashCode
13. instanceId
14. jong28
15. jungs21
16. packageInfo
17. publishedClientAppBuildNumber
18. publishedClientAppVersion
19. reportState
20. reportTitles
21. runtimeType
22. screenHeight
23. screenWidth
24. sensorState
25. userState
26. Methods
27. addCowsToCowGroup
28. createCow
29. createCowGroup
30. createDiseaseObservations
31. deleteDiseaseObservations
32. fetchAuthSession
33. fetchCowData
34. fetchCowGroups
35. fetchCows
36. fetchCowsCntByStatus
37. fetchDemographic
38. fetchDiseaseObservations
39. fetchFarms
40. fetchGateways
41. fetchReports
42. fetchReportTitles
43. fetchSensors
44. fetchUnreadReportsCnt
45. getCurrentUser
46. getDeviceId
47. isKorean
48. moveOrCopyCowsToCowGroup
49. noSuchMethod
50. registerNotificationToken
51. removeCow
52. removeCowGroup
53. removeCowsFromCowGroup
54. reqDELETE
55. reqGET
56. reqPOST
57. reqPUT
58. searchCow
59. signIn
60. signOut
61. splitJASO
62. toggleReportPinned
63. toString
64. unregisterNotificationToken
65. updateCow
66. updateCowGroup
67. updateCowGroups
68. updateDiseaseObservations
69. updateReportFeedback
70. updateReportMemo
71. userJoinFarm
72. Operators
73. operator ==