trackColor property Null safety

1. flutter_json_widgets
2. widget
3. Switch
4. trackColor property

Switch class

1. Constructors
2. Switch
3. fromJson
4. Properties
5. activeColor
6. activeThumbImage
7. activeTrackColor
8. autofocus
9. copyWith
10. dragStartBehavior
11. field
12. focusColor
13. hashCode
14. hoverColor
15. inactiveThumbColor
16. inactiveThumbImage
17. inactiveTrackColor
18. key
19. materialTapTargetSize
20. mouseCursor
21. onFocusChange
22. overlayColor
23. runtimeType
24. splashRadius
25. thumbColor
26. trackColor
27. value
28. Methods
29. map
30. mapOrNull
31. maybeMap
32. maybeWhen
33. noSuchMethod
34. toJson
35. toString
36. when
37. whenOrNull
38. Operators
39. operator ==