NIMChatroomErrors constructor Null safety

1. nim_core
2. nim_core
3. NIMChatroomErrors
4. NIMChatroomErrors constructor

NIMChatroomErrors class

1. Constructors
2. NIMChatroomErrors
3. Properties
4. hashCode
5. runtimeType
6. Methods
7. noSuchMethod
8. toString
9. Operators
10. operator ==
11. Constants
12. blacklisted
13. connectError
14. forbidden
15. illegalState
16. paramsError
17. roomNotExists
18. serverError
19. success