onFetch property Null safety

1. baguette
2. EndlessFetcher
3. EndlessFetcher<T>
4. onFetch property

EndlessFetcher class

1. Constructors
2. EndlessFetcher
3. Properties
4. fetchForIndex
5. hashCode
6. onFetch
7. runtimeType
8. Methods
9. clear
10. fetch
11. getUnits
12. noSuchMethod
13. toString
14. Operators
15. operator ==