SelectionHandler constructor Null safety

1. Flutter
2. rendering
3. SelectionHandler
4. SelectionHandler constructor

SelectionHandler class

1. Constructors
2. SelectionHandler
3. Properties
4. hashCode
5. runtimeType
6. value
7. Methods
8. addListener
9. dispatchSelectionEvent
10. getSelectedContent
11. noSuchMethod
12. pushHandleLayers
13. removeListener
14. toString
15. Operators
16. operator ==