subtitle property Null safety

1. flutter_json_widgets
2. widget
3. ExpansionTile
4. subtitle property

ExpansionTile class

1. Constructors
2. ExpansionTile
3. fromJson
4. Properties
5. backgroundColor
6. children
7. childrenPadding
8. clipBehavior
9. collapsedBackgroundColor
10. collapsedIconColor
11. collapsedShape
12. collapsedTextColor
13. controlAffinity
14. copyWith
15. expandedAlignment
16. expandedCrossAxisAlignment
17. hashCode
18. iconColor
19. initiallyExpanded
20. key
21. leading
22. maintainState
23. runtimeType
24. shape
25. subtitle
26. textColor
27. tilePadding
28. title
29. trailing
30. Methods
31. map
32. mapOrNull
33. maybeMap
34. maybeWhen
35. noSuchMethod
36. toJson
37. toString
38. when
39. whenOrNull
40. Operators
41. operator ==