NetworkEventHandler constructor Null safety

1. sendbird_sdk
2. authenticate_event_handler
3. NetworkEventHandler
4. NetworkEventHandler constructor

NetworkEventHandler class

1. Constructors
2. NetworkEventHandler
3. Properties
4. hashCode
5. runtimeType
6. Methods
7. noSuchMethod
8. onReconnect
9. toString
10. Operators
11. operator ==