flexibleSpace property Null safety

1. flutter_json_widgets
2. sliver
3. SliverAppBar
4. flexibleSpace property

SliverAppBar class

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