NIMNotificationExtraType constructor Null safety

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

NIMNotificationExtraType enum

1. Constructors
2. NIMNotificationExtraType
3. Values
4. message
5. jsonArrStr
6. Properties
7. hashCode
8. index
9. runtimeType
10. Methods
11. noSuchMethod
12. toString
13. Operators
14. operator ==
15. Constants
16. values