title: Uncaught exception of type NSException
body: <p>I'm beginning to teach myself Swift, I am truly a beginner and I am working on a calculator app just as an intro project. I keep getting a thread error and it is terminating with an uncaught exception of type NSException. I've read in several places that this is usually caused by a missing or wrong connection from the storyboard to the view controller but I triple checked all of my connections and I don't think that is the problem. Here is my code for the view controller, is there an issue with it? I followed a tutorial for the most part.</p>
  
  <pre><code>import UIKit
  
  extension String{
      var doubleValue: Double{
          if let number = NSNumberFormatter().numberFromString(self) {
              return number.doubleValue
          }
          return 0
      }
  }
  
  class ViewController: UIViewController {
  
  var isTypingNumber = false
  var firstNumber:Double? = 0
  var secondNumber:Double? = 0
  var operation = ""
  
  
  @IBOutlet var calculatorDisplay: UILabel!
  @IBAction func numberTapped(sender: AnyObject) {
      var number = sender.currentTitle
  
      if isTypingNumber {
          calculatorDisplay.text = calculatorDisplay.text! + number!!
      } else{
          calculatorDisplay.text = number
          isTypingNumber = true
      }
  }
  @IBAction func calculationTapped(sender: AnyObject) {
      isTypingNumber = false
      firstNumber = calculatorDisplay.text!.doubleValue
      operation = sender.currentTitle!!
  }
  @IBAction func equalsTapped(sender: AnyObject) {
      isTypingNumber = false
      var result = 0.0
      secondNumber = calculatorDisplay.text!.doubleValue
  
      if operation == "+"{
          result = firstNumber! + secondNumber!
      } else if operation == "-"{
          result = firstNumber! - secondNumber!
      } else if operation == "*"{
          result = firstNumber! * secondNumber!
      }
      else if operation == "/"{
          result = firstNumber! / secondNumber!
      }
  
      calculatorDisplay.text = "\(result)"
  }
  
  
  
  override func viewDidLoad() {
      super.viewDidLoad()
      // Do any additional setup after loading the view, typically from a nib.
  }
  
  override func didReceiveMemoryWarning() {
      super.didReceiveMemoryWarning()
      // Dispose of any resources that can be recreated.
  }
  
  
  }
  </code></pre>
  
  <p>I really appreciate any help I can get!</p>