isError property Null safety

1. flutter_json_widgets
2. widget
3. Checkbox
4. isError property

Checkbox class

1. Constructors
2. Checkbox
3. fromJson
4. Properties
5. activeColor
6. autofocus
7. checkColor
8. copyWith
9. field
10. fillColor
11. focusColor
12. hashCode
13. hoverColor
14. isError
15. key
16. materialTapTargetSize
17. mouseCursor
18. overlayColor
19. runtimeType
20. shape
21. side
22. splashRadius
23. tristate
24. value
25. visualDensity
26. Methods
27. map
28. mapOrNull
29. maybeMap
30. maybeWhen
31. noSuchMethod
32. toJson
33. toString
34. when
35. whenOrNull
36. Operators
37. operator ==