AVPlayer forward class declaration
I figured it out. I thought #import <AVKit/AVKit.h> was enough for AVPlayerViewController but if you want to manage the AVPlayer you also need #import <AVFoundation/AVFoundation.h>.
Have you imported AVPlayerViewController.h i.e. #import "AVPlayerViewController.h"?
A forward declaration is where the you are trying to send messages to a class that the compiler doesn't know about.