Thread 1: signal SGABRT

swift初心者です。
このたび、swiftを使って簡単な電卓を作ってみたのですが、AppDelegate.swiftの
class AppDelegate: UIResponder, UIApplicationDelegate
の行で以下のようなエラー文が発生していました。
Thread 1:signal SIGABRT

原因は何でしょうか？
下にviewController.swiftのソースコードを載せておきます

lang
1import UIKit
2
3class ViewController: UIViewController {
4    
5    override func viewDidLoad() {
6        super.viewDidLoad()
7        // Do any additional setup after loading the view, typically from a nib.
8        myButton0.addTarget(self, action: "buttonClicked", forControlEvents: .TouchUpInside)
9        myButton1.addTarget(self, action: "buttonClicked", forControlEvents: .TouchUpInside)
10        myButton2.addTarget(self, action: "buttonClicked", forControlEvents: .TouchUpInside)
11        myButton3.addTarget(self, action: "buttonClicked", forControlEvents: .TouchUpInside)
12        myButton4.addTarget(self, action: "buttonClicked", forControlEvents: .TouchUpInside)
13        myButton5.addTarget(self, action: "buttonClicked", forControlEvents: .TouchUpInside)
14        myButton6.addTarget(self, action: "buttonClicked", forControlEvents: .TouchUpInside)
15        myButton7.addTarget(self, action: "buttonClicked", forControlEvents: .TouchUpInside)
16        myButton8.addTarget(self, action: "buttonClicked", forControlEvents: .TouchUpInside)
17        myButton9.addTarget(self, action: "buttonClicked", forControlEvents: .TouchUpInside)
18        
19        myButtonPlus.addTarget(self, action: "buttonClicked", forControlEvents: .TouchUpInside)
20        myButtonMinus.addTarget(self, action: "buttonClicked", forControlEvents: .TouchUpInside)
21        myButtonMulti.addTarget(self, action: "buttonClicked", forControlEvents: .TouchUpInside)
22        myButtonDivision.addTarget(self, action: "buttonClicked", forControlEvents: .TouchUpInside)
23        myButtonEqual.addTarget(self, action: "buttonClicked", forControlEvents: .TouchUpInside)
24    }
25    
26    override func didReceiveMemoryWarning() {
27        super.didReceiveMemoryWarning()
28        // Dispose of any resources that can be recreated.
29    }
30
31    @IBOutlet weak var myLabel: UILabel!;
32    
33//    数字を表すプロパティ
34    @IBOutlet weak var myButton0: UIButton!;
35    @IBOutlet weak var myButton1: UIButton!;
36    @IBOutlet weak var myButton2: UIButton!;
37    @IBOutlet weak var myButton3: UIButton!;
38    @IBOutlet weak var myButton4: UIButton!;
39    @IBOutlet weak var myButton5: UIButton!;
40    @IBOutlet weak var myButton6: UIButton!;
41    @IBOutlet weak var myButton7: UIButton!;
42    @IBOutlet weak var myButton8: UIButton!;
43    @IBOutlet weak var myButton9: UIButton!;
44
45    @IBOutlet weak var myButtonPlus: UIButton!;//足し算のプロパティ
46    @IBOutlet weak var myButtonMinus: UIButton!;//引き算のプロパティ
47    @IBOutlet weak var myButtonMulti: UIButton!;//掛け算のプロパティ
48    @IBOutlet weak var myButtonDivision: UIButton!;//割り算のプロパティ
49    
50    @IBOutlet weak var myButtonEqual: UIButton!;
51    
52    func buttonClicked(sender: UIButton){
53        switch sender {
54        case myButtonPlus,myButtonMinus,myButtonMulti,myButtonDivision:
55            command = sender.titleLabel!.text!;
56            inputR = true;//右辺の入力に移る
57            
58        case myButtonEqual:
59            //ボタンがタップされた
60            if inputR {
61                var result: Int?;
62                    
63                switch command {
64                case "+":
65                    result = left.toInt()! + right.toInt()!;
66                case "-":
67                    result = left.toInt()! - right.toInt()!;
68                case "*":
69                    result = left.toInt()! * right.toInt()!;
70                case "/":
71                    result = left.toInt()! / right.toInt()!;
72                        
73                default :
74                    result = 0;
75                }
76                myLabel.text = "\(result)";//結果の表示
77                //次の入力のためのデータの初期化
78                left = "";
79                right = "";
80                inputR = false;
81            }
82            
83        default :
84            //ボタン0-9がタップされた時
85            if inputR {
86                //右辺入力時の動作
87                right = right + sender.titleLabel!.text!;
88                myLabel.text = right;
89            } else {
90                //左辺入力中なら
91                left = left + sender.titleLabel!.text!;
92                myLabel.text = left;
93                
94            }
95            
96        }
97        
98    }
99    
100    var right: String = "";//計算の右辺となる文字列
101    var left: String = "";//計算の左辺となる文字列
102    var inputR: Bool = false;//右辺を入力中かどうか
103    var command: String = "";//計算に使用するコマンド
104    
105
106}
107