NERtcErrorCode constructor Null safety

1. yunxin_roomkit
2. roomkit
3. NERtcErrorCode
4. NERtcErrorCode constructor

NERtcErrorCode class

1. Constructors
2. NERtcErrorCode
3. Properties
4. hashCode
5. runtimeType
6. Methods
7. noSuchMethod
8. toString
9. Operators
10. operator ==
11. Constants
12. addTrackFail
13. connectFail
14. connectionNotFound
15. createDeviceSourceFail
16. createDumpFileFail
17. deviceNotFound
18. devicePreviewAlreadyStarted
19. errorFatal
20. illegalArgument
21. illegalStatus
22. invalidDeviceSourceId
23. invalidIndex
24. invalidOperation
25. invalidParam
26. invalidRender
27. invalidState
28. invalidUserId
29. invalidVideoProfile
30. lackOfResource
31. leaveChannelForSwitch
32. mediaConnectionDisconnected
33. mediaNotStarted
34. notSupported
35. ok
36. outOfMemory
37. requestJoinRoomFail
38. reserveInvalidParameter
39. reserveMoreThanTwoUser
40. reserveNoPermission
41. reserveServerFail
42. reserveTimeout
43. roomAlreadyJoined
44. roomClosed
45. roomNotJoined
46. roomRepeatedlyLeave
47. serverKicked
48. sessionNotFound
49. signalDisconnected
50. sourceNotFound
51. startDumpFail
52. streamNotFound
53. switchChannelNotJoined
54. trackNotFound
55. transmitPending
56. uninitialized
57. userNotFound