I decided to use FirebaseAuthUI for my app and ran into something that could be helpful for others. If you go this route, you will end up with a ViewController that has a Cancel button where you don’t want one. Since this is Google’s code, it is next to impossible to track down where this button is coming from. However, there is a fix

extension FUIAuthBaseViewController{ open override func viewWillAppear(_ animated: Bool) { self.navigationItem.leftBarButtonItem = nil } }