title: Why is self.currentViewController nil in ViewDidLoad()?
body: <p>Recently changed which View in my project is the starting point (with the arrow pointing to it in IB), and now I get the following error when the app starts in the simulator:</p>
  
  <blockquote>
    <p>fatal error: unexpectedly found nil while unwrapping an Optional value</p>
  </blockquote>
  
  <p>The error occurs during the ViewDidLoad() of the view that <strong><em>used to be</em></strong> the starting point, but which isn't now.  Honestly, not even sure why it's loading, but that's not a part of Swift I know enough about to recognize if this view (also) loading at startup is expected or not.  The code where the error occurs is:</p>
  
  <pre><code>override func viewDidLoad() {
  
      print("Former First View starts DidLoad")
  
      // Code to enable the containerView to work:
      self.currentViewController = self.storyboard?.instantiateViewController(withIdentifier: "SmallViewWithinMainView")
      self.currentViewController!.view.translatesAutoresizingMaskIntoConstraints = false
      self.addChildViewController(self.currentViewController!)
      // Error occurs in the next line:
      self.addSubview(subView: self.currentViewController!.view, toView: self.SortFilterEtcContainerView)
  
      super.viewDidLoad()
  
      // ...
  </code></pre>
  
  <p>With testing, self.currentViewController is in fact now nil, but I'm unclear how to fix the situation.  Before the change, when this view was where everything started, it ran fine.  What needs to happen so that self.currentViewController is no longer nil?  </p>