NIMMessageStatus constructor Null safety

1. nim_core
2. nim_core
3. NIMMessageStatus
4. NIMMessageStatus const constructor

NIMMessageStatus enum

1. Constructors
2. NIMMessageStatus
3. Values
4. draft
5. sending
6. success
7. fail
8. read
9. unread
10. Properties
11. hashCode
12. index
13. runtimeType
14. Methods
15. noSuchMethod
16. toString
17. Operators
18. operator ==
19. Constants
20. values