onChange property Null safety

1. baguette
2. CheckWidget
3. CheckWidget
4. onChange property

CheckWidget class

1. Constructors
2. CheckWidget
3. Properties
4. check
5. hashCode
6. height
7. key
8. label
9. margin
10. onChange
11. padding
12. runtimeType
13. visibility
14. width
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 ==