title: Crash when calling getter method in Swift
body: <p>I'm doing the Stanford iOS course on iTunes U and i'm getting strange behavior when calling a getter method</p>
  
  <pre><code>import UIKit
  
  class ViewController: UIViewController {
      @IBOutlet weak var display: UILabel!
      var newNumber = true
      var operandStack = Array&lt;Double&gt;()
      var displayValue: Double{
          get{
              return NSNumberFormatter().numberFromString(display.text!)!.doubleValue
          }
          set{
              display.text = "\(newNumber)"
              newNumber = true
          }
      }
  
      @IBAction func appendDigit(sender: UIButton) {
          let digit = sender.currentTitle!
          if(newNumber){
              display.text = digit
              newNumber = false
          }else{
              display.text = display.text! + digit
          }
      }
  
      @IBAction func enter() {
          if(!newNumber){
              newNumber = true
          }
          operandStack.append(displayValue)
          println("\(operandStack)")
      }
  
  }
  </code></pre>
  
  <p>When the <code>enter()</code> method is called I get a crash with a breakpoint pointing towards the return statement in the <code>displayValue</code> get method. If I ignore the get method and call</p>
  
  <pre><code>operandStack.append(NSNumberFormatter().numberFromString(display.text!)!.doubleValue)
  </code></pre>
  
  <p>Everything works fine but I don't understand why the get method crashes, isn't it doing the exact same thing as the code above would?</p>
  
  <p>This is the full backtrace</p>
  
  <pre><code>* thread #1: tid = 0x133463, 0x000e4738 Kalkylator`Kalkylator.ViewController.displayValue.getter : Swift.Double(self=0x7986d560) + 24 at ViewController.swift:17, queue = 'com.apple.main-thread', stop reason = breakpoint 1.1
    * frame #0: 0x000e4738 Kalkylator`Kalkylator.ViewController.displayValue.getter : Swift.Double(self=0x7986d560) + 24 at ViewController.swift:17
      frame #1: 0x000e57c2 Kalkylator`Kalkylator.ViewController.enter (self=0x7986d560)() -&gt; () + 130 at ViewController.swift:39
      frame #2: 0x000e5992 Kalkylator`@objc Kalkylator.ViewController.enter (Kalkylator.ViewController)() -&gt; () + 34 at ViewController.swift:0
      frame #3: 0x01d0c7cd libobjc.A.dylib`-[NSObject performSelector:withObject:withObject:] + 84
      frame #4: 0x00b1aa90 UIKit`-[UIApplication sendAction:to:from:forEvent:] + 99
      frame #5: 0x00b1aa22 UIKit`-[UIApplication sendAction:toTarget:fromSender:forEvent:] + 64
      frame #6: 0x00c5b18a UIKit`-[UIControl sendAction:to:forEvent:] + 69
      frame #7: 0x00c5b5a7 UIKit`-[UIControl _sendActionsForEvents:withEvent:] + 598
      frame #8: 0x00c5a811 UIKit`-[UIControl touchesEnded:withEvent:] + 660
      frame #9: 0x00b72cfa UIKit`-[UIWindow _sendTouchesForEvent:] + 874
      frame #10: 0x00b737d6 UIKit`-[UIWindow sendEvent:] + 792
      frame #11: 0x00b316d1 UIKit`-[UIApplication sendEvent:] + 242
      frame #12: 0x00b41b08 UIKit`_UIApplicationHandleEventFromQueueEvent + 21484
      frame #13: 0x00b15337 UIKit`_UIApplicationHandleEventQueue + 2300
      frame #14: 0x0021706f CoreFoundation`__CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 15
      frame #15: 0x0020cb7d CoreFoundation`__CFRunLoopDoSources0 + 253
      frame #16: 0x0020c0d8 CoreFoundation`__CFRunLoopRun + 952
      frame #17: 0x0020ba5b CoreFoundation`CFRunLoopRunSpecific + 443
      frame #18: 0x0020b88b CoreFoundation`CFRunLoopRunInMode + 123
      frame #19: 0x03fa32c9 GraphicsServices`GSEventRunModal + 192
      frame #20: 0x03fa3106 GraphicsServices`GSEventRun + 104
      frame #21: 0x00b19106 UIKit`UIApplicationMain + 1526
      frame #22: 0x000eec64 Kalkylator`main + 180 at AppDelegate.swift:12
      frame #23: 0x02407ac9 libdyld.dylib`start + 1
  </code></pre>