flexibleSpace property Null safety

1. flutter_json_widgets
2. preferred_size_widget
3. AppBar
4. flexibleSpace property

AppBar class

1. Constructors
2. AppBar
3. fromJson
4. Properties
5. actions
6. automaticallyImplyLeading
7. backgroundColor
8. bottom
9. bottomOpacity
10. centerTitle
11. copyWith
12. elevation
13. excludeHeaderSemantics
14. flexibleSpace
15. forceMaterialTransparency
16. foregroundColor
17. hashCode
18. key
19. leading
20. leadingWidth
21. primary
22. runtimeType
23. scrolledUnderElevation
24. shadowColor
25. shape
26. surfaceTintColor
27. title
28. titleSpacing
29. titleTextStyle
30. toolbarHeight
31. toolbarOpacity
32. toolbarTextStyle
33. Methods
34. map
35. mapOrNull
36. maybeMap
37. maybeWhen
38. noSuchMethod
39. toJson
40. toString
41. when
42. whenOrNull
43. Operators
44. operator ==