camera callback preview didnt call

used the below thingy


/* Set the camera preview callback */
error_code = camera_set_media_packet_preview_cb(facedata.g_camera, _camera_media_packet_preview_cb, NULL);
at the guides link.
https://github.com/Samsung/tizen-docs/blob/master/docs/application/native/guides/multimedia/face-detection.md

But _camera_media_packet_preview_cb function didnt call.
