onLoading property Null safety

1. farmershands
2. custom_refresher
3. CustomRefresher
4. onLoading property

CustomRefresher class

1. Constructors
2. CustomRefresher
3. Properties
4. builder
5. cacheExtent
6. child
7. controller
8. dragStartBehavior
9. enablePullDown
10. enablePullUp
11. enableTwoLevel
12. footer
13. hashCode
14. header
15. key
16. onLoading
17. onRefresh
18. onTwoLevel
19. physics
20. primary
21. reverse
22. runtimeType
23. scrollController
24. scrollDirection
25. semanticChildCount
26. Methods
27. build
28. createElement
29. debugDescribeChildren
30. debugFillProperties
31. noSuchMethod
32. toDiagnosticsNode
33. toString
34. toStringDeep
35. toStringShallow
36. toStringShort
37. Operators
38. operator ==