columns property Null safety

1. flutter_json_widgets
2. widget
3. DataTable
4. columns property

DataTable class

1. Constructors
2. DataTable
3. fromJson
4. Properties
5. border
6. checkboxHorizontalMargin
7. clipBehavior
8. columns
9. columnSpacing
10. copyWith
11. dataRowColor
12. dataRowHeight
13. dataTextStyle
14. decoration
15. dividerThickness
16. hashCode
17. headingRowColor
18. headingRowHeight
19. headingTextStyle
20. horizontalMargin
21. key
22. onDeselectAll
23. onSelectAll
24. rows
25. runtimeType
26. showBottomBorder
27. showCheckboxColumn
28. sortAscending
29. sortColumnIndex
30. Methods
31. map
32. mapOrNull
33. maybeMap
34. maybeWhen
35. noSuchMethod
36. toJson
37. toString
38. when
39. whenOrNull
40. Operators
41. operator ==