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When PageController trying to present it's content I'm encountering following message in console: "[Assert] Window should be nonnil here unless a subclass is mistakenly sending this to a child when no window can be found". App is not crashing it's just not showing content it's supposed to show in PageController. Code seems fine to me: class RulesPageViewController: UIPageViewController { var rulesDelegate: RulesPageVeiwControllerDelegate? var pageHeadings = ["1st page","2nd","3rd"] var currentIndex = 0 override func viewDidLoad() { super.viewDidLoad() self.dataSource = self self.delegate = self if let startingViewController = contentViewController(at: 0) { self.setViewControllers([startingViewController], direction: .forward, animated: false, completion: nil) } } func contentViewController(at index: Int)-> PageContentViewController? { if(index < 0 || index >= pageHeadings.count) { return nil } if let pageContentViewController = UIStoryboard(name: "GameRules", bundle: nil).instantiateViewController(withIdentifier: "PageContentViewController") as? PageContentViewController{ pageContentViewController.index = index return pageContentViewController } return nil } } extension RulesPageViewController: UIPageViewControllerDelegate { func pageViewController(_ pageViewController: UIPageViewController, didFinishAnimating finished: Bool, previousViewControllers: [UIViewController], transitionCompleted completed: Bool) { if completed { if let contentViewController = pageViewController.viewControllers?.first as? PageContentViewController { currentIndex = contentViewController.index rulesDelegate?.didUpdatePageIndex(currentIndex: currentIndex) } } } } extension RulesPageViewController: UIPageViewControllerDataSource { func pageViewController(_ pageViewController: UIPageViewController, viewControllerBefore viewController: UIViewController) -> UIViewController? { var index = (viewController as! PageContentViewController).index index -= 1 return contentViewController(at: index) } func pageViewController(_ pageViewController: UIPageViewController, viewControllerAfter viewController: UIViewController) -> UIViewController? { var index = (viewController as! PageContentViewController).index index += 1 return contentViewController(at: index) } }