画像をタップした際のタイマー停止

画像をタップしたときにタイマーを停止させたい

ここに質問の内容を詳しく書いてください。
タイマーで画像を切り替える機能をつけたスライドショーを作っています。
画像をタップした際に画面を切り替えると同時にタイマーをストップさせたいのですがうまくいきません。
コードがうまく動作しているのかprint文を用いて確認してみたところうまくprintが動作されませんでした。

該当のソースコード

Swift
1import UIKit
2
3class ViewController: UIViewController {
4    
5    @IBOutlet weak var button: UIButton!
6    
7    @IBOutlet weak var topImage: UIImageView!
8    
9    @IBOutlet weak var backButton: UIButton!
10    
11    @IBOutlet weak var nextButton: UIButton!
12    
13    var imageIndex = 0
14    
15    var timer: Timer!
16    var timer_sec: Float = 0
17    
18    let images = [UIImage(named: "imageA"), UIImage(named: "imageB"), UIImage(named: "imageC"),UIImage(named: "imageD")]
19    
20    override func viewDidLoad() {
21        super.viewDidLoad()
22        // Do any additional setup after loading the view, typically from a nib.
23        
24        let imageView = images[0]
25        
26        topImage.image = imageView
27    }
28
29    override func didReceiveMemoryWarning() {
30        super.didReceiveMemoryWarning()
31        // Dispose of any resources that can be recreated.
32    }
33    
34    @IBAction func backImage(_ sender: Any) {
35        if imageIndex == 0 {
36            imageIndex = 3
37        } else {
38            imageIndex -= 1
39        }
40        topImage.image = images[imageIndex]
41    }
42    
43    @IBAction func nextImage(_ sender: Any) {
44        if imageIndex == 3 {
45            imageIndex = 0
46        } else {
47            imageIndex += 1
48        }
49        topImage.image = images[imageIndex]
50    }
51    
52    @IBAction func startStop(_ sender: Any) {
53        if self.timer == nil {
54            self.timer = Timer.scheduledTimer(timeInterval: 2, target: self, selector: #selector(onTimer(_:)), userInfo: nil, repeats: true)
55                button.setTitle("一時停止", for: .normal)
56                backButton.isEnabled = false
57                nextButton.isEnabled = false
58        } else {
59            if self.timer == nil {
60                self.timer = Timer.scheduledTimer(timeInterval: 2, target: self, selector: #selector(onTimer(_:)), userInfo: nil, repeats: true)
61            } else {
62                    if timer.isValid == true {
63                        timer.invalidate()
64                        button.setTitle("再生", for: .normal)
65                        backButton.isEnabled = true
66                        nextButton.isEnabled = true
67                    } else {
68                        self.timer = Timer.scheduledTimer(timeInterval: 2, target: self, selector: #selector(onTimer(_:)), userInfo: nil, repeats: true)
69                        button.setTitle("一時停止", for: .normal)
70                        backButton.isEnabled = false
71                        nextButton.isEnabled = false
72                    }
73            }
74        }
75    }
76    
77        @objc func onTimer(_ timer: Timer) {
78            if imageIndex == 2 {
79                imageIndex = 0
80            } else {
81                imageIndex += 1
82            }
83            topImage.image = images[imageIndex]
84        }
85    
86    @IBAction func tapImage(_ sender: Any) {
87        if self.timer != nil {
88　　　　　　  print("タイマー停止")
89            button.setTitle("再生", for: .normal)
90            backButton.isEnabled = true
91            nextButton.isEnabled = true
92            timer.invalidate()
93            self.timer = nil
94        }
95        performSegue(withIdentifier: "toSecondVC", sender: nil)
96    }
97    
98    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
99        let secondViewController:SecondViewController = segue.destination as! SecondViewController
100        
101        secondViewController.image = topImage.image!
102    }
103    
104    @IBAction func unwind(_ segue: UIStoryboardSegue) {
105    }
106    
107}

import UIKit

class SecondViewController: UIViewController {
    
    @IBOutlet weak var imageView: UIImageView!
    
    var image:UIImage = UIImage()
    
    override func viewDidLoad() {
        super.viewDidLoad()

        // Do any additional setup after loading the view.
        imageView.image = image
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }

}