Functions that delay adding views to a navigation controller until they are removed from their parent view controllers.

DelayedAddition.swift

func eventuallyAdd(_ viewControllers: [UIViewController], to navigationController: UINavigationController) {
    if canAdd(viewControllers, to: navigationController) {
        navigationController.viewControllers += viewControllers
    } else {
            
        // SHOW PLACEHOLDER CONTENT HERE IF NEEDED
            
        DispatchQueue.main.async {
            eventuallyAdd(viewControllers, to: navigationController)
        }
    }
}

func canAdd(_ viewControllers: [UIViewController], to: UIViewController? = nil) -> Bool {
    for viewController in viewControllers where !viewController.canBeAdded(to: to) {
        return false
    }
    return true
}

extension UIViewController {
    @nonobjc func canBeAdded(to: UIViewController? = nil) -> Bool {
        guard parent == nil && isViewLoaded && view.window != nil else { return true }
        guard let parentViewController: UIViewController = view.superview?.nextViewController else { return true }
        return parentViewController == nil || parentViewController == to
    }
}

public extension UIResponder {
    @nonobjc var nextViewController: UIViewController? {
        guard let next = self.next else { return nil }
        if let next = next as? UIViewController {
            return next
        } else {
            return next.nextViewController
        }
    }
}