Login page doesn't work fully it works when segue connected but doesn't check if user is registered or not - ios

Question

Login page doesn't really check for if user is stored in core data and will just go ahead with the segue. It should login only if user is registered in core data else will input error and to go register but register seems to work correctly tho I followed some guides online
I have attached the code please check and give me some suggestion to achieve my task
LoginVC
import UIKit
import CoreData
class LoginVC: UIViewController {
#IBOutlet weak var username: UITextField!
#IBOutlet weak var password: UITextField!
var credits = 200.0
var context = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view
fetchData()
}
#IBAction func login(_ sender: Any) {
for acc in userList {
if acc.username == username.text && acc.password == password.text {
currentUser = username.text!
try! context.save()
//performSegue(withIdentifier: "DisplayShop1", sender: nil)
let storyBoard : UIStoryboard = UIStoryboard(name: "Main", bundle: nil)
let next = storyBoard.instantiateViewController(identifier: "DisplayShop1") as! ListingShopVC
next.modalPresentationStyle = .fullScreen
self.present(next, animated: true, completion: nil)
}
else if username.text != acc.username || password.text != acc.password || password.text == "" || username.text == "" {
let alert = UIAlertController(title: "Alert", message: "Please enter the correct credentials", preferredStyle: .alert)
alert.addAction(UIAlertAction(title: "Dismiss", style: .default, handler: nil))
present(alert, animated: true, completion: nil)
}
}
}
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
if segue.identifier == "hellothere" {
let ve = segue.destination as! ListingShopVC
ve.creditsss = credits
}
}
func fetchData(){
userList = try! context.fetch(User.fetchRequest())
}
}
RegisterVC
import UIKit
import CoreData
class RegisterVC: UIViewController {
var context = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
var userList: String = ""
#IBOutlet weak var username: UITextField!
#IBOutlet weak var cpassword: UITextField!
#IBOutlet weak var password: UITextField!
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view.
}
#IBAction func register(_ sender: Any) {
if username.text != ""{
if password.text != "" {
if password.text == cpassword.text {
let newu = User(context: context)
newu.username = username.text
newu.password = password.text
newu.credit = 200.0
try! context.save()
self.navigationController?.popViewController(animated: true)
}
else {
let alert = UIAlertController(title: "Alert", message: "Please enter values correctly", preferredStyle: .alert)
alert.addAction(UIAlertAction(title: "Dismiss", style: .default, handler: nil))
present(alert, animated: true, completion: nil)*/
}
}
}
}