title: UITableview lost reference after pop view controller
body: <p>For example i have  <code>ViewController1</code> with <code>UITableview</code> object and data.Then i will push to another controller from <code>ViewController1</code> to <code>ViewController2</code> .</p>
  
  <p>My problem is when i am trying to pop from <code>ViewController2</code> to <code>ViewController1</code> then <code>UITableview</code> object return <code>nil</code> and my application is crashed.This problem often happen.</p>
  
  <p>I am using storyboard for assign <code>UITableview</code>. And I am using <code>swift 2.0</code> as programming language.</p>
  
  <p>Below is my source code where my app is crashed.</p>
  
  <pre><code>  override func viewDidAppear(animated: Bool)
  {
      super.viewDidAppear(true)
      btnRightBarBtnItem.image = UIImage(named : "Home")
      btnRightBarBtnItem.target = self
      btnRightBarBtnItem.action = #selector(self.btnHomeAction)
  
      refreshControl.attributedTitle = NSAttributedString(string: "Pull to refresh")
      refreshControl.addTarget(self, action: #selector(SelectPackingItemsViewController.refresh(_:)), forControlEvents: UIControlEvents.ValueChanged)
       packingItemsTableView.addSubview(refreshControl)   
  }
  </code></pre>
  
  <p>App is crashed on "packingItemsTableView.addSubview(refreshControl)" this line.</p>
  
  <p>It returns in log 
  (lldb) po self.packingItemsTableView!
  fatal error: unexpectedly found nil while unwrapping an Optional value</p>
  
  <p>refresh control is nothing but "UIRefreshControl() object" which does not nil.</p>