selectedIndex property Null safety

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

NavigationBar class

1. Constructors
2. NavigationBar
3. fromJson
4. Properties
5. animationDuration
6. backgroundColor
7. copyWith
8. destinations
9. elevation
10. hashCode
11. height
12. indicatorColor
13. indicatorShape
14. key
15. labelBehavior
16. onDestinationSelected
17. runtimeType
18. selectedIndex
19. shadowColor
20. surfaceTintColor
21. Methods
22. map
23. mapOrNull
24. maybeMap
25. maybeWhen
26. noSuchMethod
27. toJson
28. toString
29. when
30. whenOrNull
31. Operators
32. operator ==