onOpened property Null safety

1. flutter_json_widgets
2. widget
3. PopupMenuButton
4. onOpened property

PopupMenuButton class

1. Constructors
2. PopupMenuButton
3. fromJson
4. Properties
5. child
6. clipBehavior
7. color
8. constraints
9. copyWith
10. elevation
11. enabled
12. enableFeedback
13. field
14. hashCode
15. icon
16. iconSize
17. initialValue
18. items
19. key
20. offset
21. onCanceled
22. onOpened
23. padding
24. position
25. runtimeType
26. shadowColor
27. shape
28. splashRadius
29. surfaceTintColor
30. tooltip
31. Methods
32. map
33. mapOrNull
34. maybeMap
35. maybeWhen
36. noSuchMethod
37. toJson
38. toString
39. when
40. whenOrNull
41. Operators
42. operator ==