SettingsViewController.swift

@IBAction func didTapSave(_ sender: Any) {
    guard let user = Auth.auth().currentUser else {
        showErrorAlert(error: nil)
        return
    }

    let newName = nameTextField.text
    let changeRequest = user.createProfileChangeRequest()
    changeRequest.displayName = newName
    changeRequest.commitChanges { (error) in
        if let error = error {
            self.showErrorAlert(error: error)
        } else {
            let ref = Database.database().reference()
            ref.child("users/\(user.uid)/username").setValue(newName?.lowercased())
            ref.child("users/\(user.uid)/displayname").setValue(newName)

            let alert = UIAlertController(title: "Success", message: "Your changes have been saved.", preferredStyle: .alert)
            self.present(alert, animated: true)
            DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + 1, execute: {
                alert.dismiss(animated: true, completion: {
                    self.navigationController?.popViewController(animated: true)
                })
            })
        }
    }
}

func showErrorAlert(error: Error?) {
    if let error = error {
        print(error.localizedDescription)
    }

    let alert = UIAlertController(title: "Error", message: "Something went wrong. Please try again.", preferredStyle: .alert)
    self.present(alert, animated: true)
    DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + 1, execute: {
        alert.dismiss(animated: true)
    })
}