present view controller on entering background

background-vc.swift

    func frontViewController(window: UIWindow) -> UIViewController {
        if let rootVC = window.rootViewController {
            var frontVC = rootVC

            while (true) {
                if let vc = frontVC.presentedViewController { frontVC = vc }
                else { break }
            }

            return frontVC
        }

        return window.rootViewController!
    }

    func applicationDidEnterBackground(application: UIApplication) {
        if let window = application.windows[0] as? UIWindow,
            let model = self.model,
            let vc = storyboard.instantiateViewControllerWithIdentifier("LoginWithStoredAuthVC") as? LoginWithStoredAuthVC {
                window.rootViewController?.view.endEditing(true)
                
                let frontVC = self.frontViewController(window)
                if frontVC is LoginWithStoredAuthVC || frontVC is LoginWithUserAndPassVC { return }
                
                vc.model = model
                vc.modalTransitionStyle = UIModalTransitionStyle.CoverVertical
                vc.modalPresentationStyle = UIModalPresentationStyle.FullScreen
                
                frontVC.presentViewController(vc, animated: false, completion: nil)
        }
    }