itemExtent property Null safety

1. flutter_json_widgets
2. widget
3. ListView
4. itemExtent property

ListView class

1. Constructors
2. ListView
3. fromJson
4. Properties
5. addAutomaticKeepAlives
6. addRepaintBoundaries
7. addSemanticIndexes
8. cacheExtent
9. children
10. clipBehavior
11. copyWith
12. hashCode
13. itemExtent
14. key
15. padding
16. physics
17. primary
18. prototypeItem
19. restorationId
20. reverse
21. runtimeType
22. scrollDirection
23. semanticChildCount
24. shrinkWrap
25. Methods
26. map
27. mapOrNull
28. maybeMap
29. maybeWhen
30. noSuchMethod
31. toJson
32. toString
33. when
34. whenOrNull
35. Operators
36. operator ==