title: Cannot convert value of type '()' to expected argument type '[UIViewController]?
body: <p>In my app I was trying to do something like this:</p>
  
  <pre><code>class tabBarController : UITabBarController {
  
      ...
  
      func example (){
          let navController : UINavigationController = UINavigationController(rootViewController: calendarVC) //calendarVC is a UIViewController
  
          guard var VCs = self.viewControllers as [UIViewController]? else{
              self.setViewControllers([navController], animated: false)
              return
          }
          self.setViewControllers(VCs.append(navController), animated: false)
      }
  
      ...
  
  }
  </code></pre>
  
  <p>But in the last line of the function I got an error:</p>
  
  <blockquote>
    <p>Cannot convert value of type '()' to expected argument type '[UIViewController]?</p>
  </blockquote>
  
  <p>How can I fix this? Thanks.</p>