title: unexpectedly found nil while unwrapping
body: <p>My code is crashing when i am unwrapping Double(display.text!)! 
  i put an if condition but it didn't work </p>
  
  <pre><code>@IBAction private func buttonpress(sender: UIButton)
  {
     let digit = sender.currentTitle!
      if userisinthemiddle
      {
          let currenttext = display.text!
          display.text = currenttext + digit
      }
      else
      {
          display.text = digit
      }
      userisinthemiddle = true
  }
  </code></pre>
  
  <p>till here it is working properly but when i try to make it as a property it is not working </p>
  
  <pre><code>var DisplayValue : Double
  {
      get
      {
           return Double(display.text!)!  // thread 1 
        }
  
      set
      {
          display.text = String(newValue)
      }
  }
  </code></pre>