params property Null safety

1. flutter_json_widgets
2. callback
3. UrlCallback
4. params property

UrlCallback class

1. Constructors
2. UrlCallback
3. fromJson
4. Properties
5. copyWith
6. hashCode
7. params
8. path
9. runtimeType
10. url
11. Methods
12. map
13. mapOrNull
14. maybeMap
15. maybeWhen
16. noSuchMethod
17. toJson
18. toString
19. when
20. whenOrNull
21. Operators
22. operator ==