onWrite property Null safety

1. baguette
2. MessageWriterWidget
3. MessageWriterWidget
4. onWrite property

MessageWriterWidget class

1. Constructors
2. MessageWriterWidget
3. Properties
4. hashCode
5. key
6. margin
7. onWrite
8. padding
9. runtimeType
10. Methods
11. createElement
12. createState
13. debugDescribeChildren
14. debugFillProperties
15. noSuchMethod
16. toDiagnosticsNode
17. toString
18. toStringDeep
19. toStringShallow
20. toStringShort
21. Operators
22. operator ==