itemExtent property Null safety

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

ListViewCustom class

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