Preview constructor Null safety

1. theta
2. theta
3. Preview
4. Preview constructor

Preview class

1. Constructors
2. Preview
3. Properties
4. hashCode
5. runtimeType
6. Methods
7. noSuchMethod
8. toString
9. Operators
10. operator ==
11. Static properties
12. keepRunning
13. Static methods
14. getFrames
15. getLivePreview
16. stopPreview