ChatMultiDeviceListener constructor Null safety

1. agora_chat_sdk
2. agora_chat_sdk
3. ChatMultiDeviceListener
4. ChatMultiDeviceListener constructor

ChatMultiDeviceListener class

1. Constructors
2. ChatMultiDeviceListener
3. Properties
4. hashCode
5. runtimeType
6. Methods
7. noSuchMethod
8. onChatThreadEvent
9. onContactEvent
10. onGroupEvent
11. toString
12. Operators
13. operator ==