title: material components tab bar controller error
body: <p>Receiving error: "Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Invalid item'" when selecting a tab bar item using MDCTabBarViewController. View controller loads but crashes right after selecting the second tab. </p>
  
  <pre><code>    class BusTabBarController: MDCTabBarViewController {
  
          override func viewDidLoad() {
              super.viewDidLoad()
  
              view.backgroundColor = .white
              loadTabBar()
          }
  
          func loadTabBar() {
  
              let firstVC = BusStopFlexibleHeaderContainerViewController()
              let secondVC = BookmarksFlexibleHeaderContainerViewController()
              let thirdVC = UIViewController()
  
              let viewControllersArray = [firstVC, secondVC, thirdVC]
              viewControllers = viewControllersArray
  
              let childVC = viewControllers.first
              selectedViewController = childVC
  
              tabBar?.delegate = self
  
              tabBar?.items = [UITabBarItem(title: "Recents", image: nil, tag: 0),
                           UITabBarItem(title: "Favorites", image: nil, tag: 0),                                                           
                           UITabBarItem(title: "YAY", image: nil, tag: 2)]
  
              tabBar?.selectedItem = tabBar?.items.first
  
              tabBar?.backgroundColor = MDCPalette.grey.tint900
              tabBar?.selectedItemTintColor = .white
              tabBar?.unselectedItemTintColor = MDCPalette.grey.tint400
              tabBar?.inkColor = MDCPalette.blueGrey.tint100
  
          }
  </code></pre>
  
  <p>Any help is appreciated. Thanks!</p>