darkTheme property Null safety

1. flutter_json_widgets
2. widget
3. MaterialApp
4. darkTheme property

MaterialApp class

1. Constructors
2. MaterialApp
3. fromJson
4. Properties
5. checkerboardOffscreenLayers
6. checkerboardRasterCacheImages
7. color
8. copyWith
9. darkTheme
10. debugShowCheckedModeBanner
11. debugShowMaterialGrid
12. hashCode
13. highContrastDarkTheme
14. highContrastTheme
15. home
16. initialRoute
17. key
18. locale
19. restorationScopeId
20. routes
21. runtimeType
22. showPerformanceOverlay
23. showSemanticsDebugger
24. supportedLocales
25. theme
26. themeAnimationCurve
27. themeAnimationDuration
28. themeMode
29. title
30. useInheritedMediaQuery
31. Methods
32. map
33. mapOrNull
34. maybeMap
35. maybeWhen
36. noSuchMethod
37. toJson
38. toString
39. when
40. whenOrNull
41. Operators
42. operator ==