highlightColor property Null safety

1. flutter_json_widgets
2. widget
3. MaterialButton
4. highlightColor property

MaterialButton class

1. Constructors
2. MaterialButton
3. fromJson
4. Properties
5. animationDuration
6. autofocus
7. child
8. clipBehavior
9. color
10. colorBrightness
11. copyWith
12. disabledColor
13. disabledElevation
14. disabledTextColor
15. elevation
16. enableFeedback
17. focusColor
18. focusElevation
19. hashCode
20. height
21. highlightColor
22. highlightElevation
23. hoverColor
24. hoverElevation
25. key
26. materialTapTargetSize
27. minWidth
28. mouseCursor
29. onLongPress
30. onPressed
31. padding
32. runtimeType
33. shape
34. splashColor
35. textColor
36. visualDensity
37. Methods
38. map
39. mapOrNull
40. maybeMap
41. maybeWhen
42. noSuchMethod
43. toJson
44. toString
45. when
46. whenOrNull
47. Operators
48. operator ==