onFetch property Null safety

1. baguette
2. MessengerScreenWidget
3. MessengerScreenWidget
4. onFetch property

MessengerScreenWidget class

1. Constructors
2. MessengerScreenWidget
3. Properties
4. data
5. hashCode
6. isArchives
7. itemMargin
8. itemPadding
9. key
10. listMargin
11. listPadding
12. onArchive
13. onFetch
14. onPress
15. onReport
16. onUnarchive
17. runtimeType
18. Methods
19. createElement
20. createState
21. debugDescribeChildren
22. debugFillProperties
23. getName
24. noSuchMethod
25. toDiagnosticsNode
26. toString
27. toStringDeep
28. toStringShallow
29. toStringShort
30. Operators
31. operator ==