startMedia method Null safety

1. baguette
2. MediaViewerWidget
3. MediaViewerWidget
4. startMedia method

MediaViewerWidget class

1. Constructors
2. MediaViewerWidget
3. Properties
4. allowFullscreen
5. allowVolume
6. autoPlay
7. background
8. fitCenter
9. hashCode
10. headers
11. height
12. key
13. runtimeType
14. uri
15. width
16. Methods
17. createElement
18. createState
19. debugDescribeChildren
20. debugFillProperties
21. isVideo
22. noSuchMethod
23. startMedia
24. stopMedia
25. toDiagnosticsNode
26. toString
27. toStringDeep
28. toStringShallow
29. toStringShort
30. Operators
31. operator ==