InboxMessage_Payload constructor Null safety

1. spine_client
2. inbox.pb
3. InboxMessage_Payload
4. InboxMessage_Payload const constructor

InboxMessage_Payload enum

1. Constructors
2. InboxMessage_Payload
3. Values
4. event
5. command
6. notSet
7. Properties
8. hashCode
9. index
10. runtimeType
11. Methods
12. noSuchMethod
13. toString
14. Operators
15. operator ==
16. Constants
17. values