title: Xcode gives an error "Unrecognized selector sent to instance”
body: <p>I try to make an calculator app, just to get closer to the Swift syntax. 
  Everything worked just fine, until I tried to implement sqrt function which I make with switch statement for usability of the other symbols I will include. </p>
  
  <p>Suddenly I am getting SIGABRD error after including a performOperation method.
  Am I missing something? I really cannot find anything.</p>
  
  <pre><code>import UIKit
  
  class ViewController: UIViewController {
  
      @IBOutlet weak var display: UILabel!
  
      @IBAction func touchDigit(_ sender: UIButton) {
          let digit = sender.currentTitle!
  
          if display.text! == "0" {
              display.text! = digit
          } else {
              display.text! = display.text! + digit
          }
      }
  
      var displayValue: Double {
          get {
              return Double(display.text!)!
          }
  
          set {
              display.text = String(newValue)
          }
      }
  
      @IBAction func performOperation(_ sender: UIButton) {
          if let mathematicalSymbol = sender.currentTitle {
              switch mathematicalSymbol {
              case "√":
                  print("Hello")
              default:
                  break
              }
          }
      }
  }
  </code></pre>