I have a function to start and stop background music during a card game , the app builds and works ok but when the music button is pressed the above error comes up in app delegate. From what I can find the error is something to do with the line containing #selector. Any suggestions on how to get the error resolved would be appreciated.

   func setupButtons(){
   button = UIButton()
    button.frame = CGRect(x: 0, y: 0, width: self.view!.frame.width / 3, height: 60)
    button.center = CGPoint(x:205, y:80)
    button.setTitle("music", for: UIControl.State.normal)
    button.backgroundColor = UIColor.darkGray
    button.layer.borderWidth = 5
    button.layer.cornerRadius = 10
    button.addTarget(self, action: #selector(MusicPlayer.didTapButton), for: UIControl.Event.touchUpInside)

    self.view!.addSubview(button)

       }

       // play background music

       @objc func didTapButton(){
    if let player = audioPlayer, player.isPlaying{
        //stop playback
        button.setTitle("play", for: .normal)
        player.pause()
    }
    else {
        //set up player, and play
        button.setTitle("pause", for: .normal)
        let urlString = Bundle.main.path(forResource: "playlistbj", ofType: "mp3")

        do{
            try AVAudioSession.sharedInstance().setMode(.default)
            try AVAudioSession.sharedInstance().setActive(true, options: .notifyOthersOnDeactivation)

            guard let urlString = urlString else{
                return
            }
           audioPlayer = try AVAudioPlayer(contentsOf: URL(fileURLWithPath: urlString))

            guard let player = audioPlayer else{
                return
            }

            player.play()
        }
        catch {
            print ("oops")
        }
    }

}

thanks for the help, that has ticked one box of on my list.