App constructor Null safety

1. next_photo
2. app
3. App
4. App constructor

App class

1. Constructors
2. App
3. Properties
4. hashCode
5. key
6. runtimeType
7. Methods
8. build
9. createElement
10. debugDescribeChildren
11. debugFillProperties
12. noSuchMethod
13. toDiagnosticsNode
14. toString
15. toStringDeep
16. toStringShallow
17. toStringShort
18. Operators
19. operator ==