margin property Null safety

1. flutter_json_widgets
2. widget
3. Container
4. margin property

Container class

1. Constructors
2. Container
3. fromJson
4. Properties
5. alignment
6. child
7. clipBehavior
8. color
9. constraints
10. copyWith
11. decoration
12. foregroundDecoration
13. hashCode
14. height
15. key
16. margin
17. padding
18. runtimeType
19. transform
20. transformAlignment
21. width
22. Methods
23. map
24. mapOrNull
25. maybeMap
26. maybeWhen
27. noSuchMethod
28. toJson
29. toString
30. when
31. whenOrNull
32. Operators
33. operator ==