onFetch property Null safety

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

MessageListWidget class

1. Constructors
2. MessageListWidget
3. Properties
4. conversation
5. hashCode
6. itemMargin
7. itemPadding
8. key
9. margin
10. onActions
11. onFetch
12. onPress
13. padding
14. runtimeType
15. Methods
16. createElement
17. createState
18. debugDescribeChildren
19. debugFillProperties
20. noSuchMethod
21. toDiagnosticsNode
22. toString
23. toStringDeep
24. toStringShallow
25. toStringShort
26. Operators
27. operator ==