SessionEventHandler constructor Null safety

1. sendbird_sdk
2. session_event_handler
3. SessionEventHandler
4. SessionEventHandler constructor

SessionEventHandler class

1. Constructors
2. SessionEventHandler
3. Properties
4. hashCode
5. runtimeType
6. Methods
7. noSuchMethod
8. onSessionClosed
9. onSessionError
10. onSessionExpired
11. onSessionRefreshed
12. onSessionTokenRequired
13. toString
14. Operators
15. operator ==