mediaViewControllerAtIndex.swift

private func mediaViewControllerAtIndex(_ index: Int) -> MediaViewController? {
	// returns nil if the index is out of bounds, this way the app won't crash.
	// You can also put a print statement before you return.
	guard (0...mediaItems.count).contains(index) else { return nil }

	let item = mediaItems[index]

	switch item.mediaType {  
	case .video:  
		return VideoViewController(mediaItem: item)  
	case .photo:  
		return PhotoViewController(mediaItem: item)
	}
}