accepted constant Null safety

1. wings_data_model
2. wings_data_model
3. NeedInvitationStatus
4. accepted constant

NeedInvitationStatus class

1. Properties
2. hashCode
3. name
4. runtimeType
5. Methods
6. noSuchMethod
7. serialize
8. toString
9. Operators
10. operator ==
11. Static properties
12. serializer
13. values
14. Static methods
15. deserialize
16. valueOf
17. Constants
18. accepted
19. canceled
20. constantName
21. sent
22. supportOffered