onClose property Null safety

1. baguette
2. MediaScreenWidget
3. MediaScreenWidget
4. onClose property

MediaScreenWidget class

1. Constructors
2. MediaScreenWidget
3. Properties
4. data
5. hashCode
6. headers
7. key
8. onClose
9. runtimeType
10. uri
11. Methods
12. createElement
13. createState
14. debugDescribeChildren
15. debugFillProperties
16. getName
17. noSuchMethod
18. toDiagnosticsNode
19. toString
20. toStringDeep
21. toStringShallow
22. toStringShort
23. Operators
24. operator ==