onTag property Null safety

1. baguette
2. TagListWidget
3. TagListWidget
4. onTag property

TagListWidget class

1. Constructors
2. TagListWidget
3. Properties
4. allowEdition
5. corner
6. hashCode
7. key
8. label
9. margin
10. onEdit
11. onTag
12. padding
13. runtimeType
14. tags
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 ==