MessageDirection constructor Null safety

1. agora_chat_sdk
2. agora_chat_sdk
3. MessageDirection
4. MessageDirection const constructor

MessageDirection enum

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