selectedIndex property Null safety

1. flutter_json_widgets
2. widget
3. NavigationRail
4. selectedIndex property

NavigationRail class

1. Constructors
2. NavigationRail
3. fromJson
4. Properties
5. backgroundColor
6. copyWith
7. destinations
8. elevation
9. extended
10. groupAlignment
11. hashCode
12. indicatorColor
13. indicatorShape
14. key
15. labelType
16. leading
17. minExtendedWidth
18. minWidth
19. onDestinationSelected
20. runtimeType
21. selectedIconTheme
22. selectedIndex
23. selectedLabelTextStyle
24. trailing
25. unselectedIconTheme
26. unselectedLabelTextStyle
27. useIndicator
28. Methods
29. map
30. mapOrNull
31. maybeMap
32. maybeWhen
33. noSuchMethod
34. toJson
35. toString
36. when
37. whenOrNull
38. Operators
39. operator ==