NEPreviewRoomContext constructor Null safety

1. netease_roomkit
2. netease_roomkit
3. NEPreviewRoomContext
4. NEPreviewRoomContext constructor

NEPreviewRoomContext class

1. Constructors
2. NEPreviewRoomContext
3. Properties
4. hashCode
5. previewController
6. runtimeType
7. Methods
8. addEventCallback
9. noSuchMethod
10. removeEventCallback
11. toString
12. Operators
13. operator ==