UserEventHandler constructor Null safety

1. sendbird_sdk
2. user_event_handler
3. UserEventHandler
4. UserEventHandler constructor

UserEventHandler class

1. Constructors
2. UserEventHandler
3. Properties
4. hashCode
5. runtimeType
6. Methods
7. noSuchMethod
8. onFriendsDiscovered
9. onTotalUnreadMessageCountUpdated
10. toString
11. Operators
12. operator ==