title: Pass data to another view controller in navigation controller
body: <p>I have two view controllers (fitstViewController.swift and secondViewController.swift) in navigation controler.</p>
  
  <p>My idea is that
  In firstViewController there are a button and a textFIeld. When I click the button, value in the textField is passed to secondViewController.</p>
  
  <p>However it makes an error "Thread 1: signal SIGABRT" in the first controller.</p>
  
  <p>Here is the code in the first controller..</p>
  
  <pre><code>override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
      let destination = segue.destinationViewController as! UINavigationController
  
      let detailController = destination.topViewController as! secondViewController
  
      detailController.stockSymbol = textField.text
  }
  </code></pre>
  
  <p>I added just one line in the second controller..</p>
  
  <pre><code>    var stockSymbol:String!
  </code></pre>
  
  <p>How can it be solved?</p>
  
  <p>Thank you in advance!</p>