CallbackManager constructor Null safety

1. Flutter
2. common
3. CallbackManager
4. CallbackManager constructor

CallbackManager class

1. Constructors
2. CallbackManager
3. Properties
4. hashCode
5. runtimeType
6. Methods
7. callback
8. cleanup
9. convertFlutterSurfaceToImage
10. noSuchMethod
11. takeScreenshot
12. toString
13. Operators
14. operator ==