How to check if user already register or not while registration in swift

I am trying to check if user is already registered or not in my app while registration. for first time for every successful registration i am getting uid in LoginViewcontroller url and i am saving that uid in keychainWrapper. and i am retrieving uid value in RegistrationViewController regesterservice But when i am trying to check that uid i am getting nil all the time why?.. how to check if user is already registred or not. please help me in the code.

In LoginViewcintroller loginnservice i am saving uid value:

var Uid = json["id"] as? String
KeychainWrapper.standard.set(emailL ?? "", forKey: "user_email")
KeychainWrapper.standard.set(self.Uid!, forKey: "Uid")

here in RegisterViewController registerservice retrieving uid value and for already reagistred person also getting nil in uid.. why?

do{
let userId: String? = KeychainWrapper.standard.string(forKey: "Uid")
print("login userid \(userId)")
if userId != nil{
    AlertFun.ShowAlert(title: "Title", message: "user exist", in: self)
}
else{
    let json = try JSONSerialization.jsonObject(with: data, options: .mutableContainers) as! [String: Any]
    print("go to otp service!!!")
    self.otpField = json["otp"] as? Int
}

}catch{
print("error")
}

How can i get already registred person uid in rgistrservice.

Please help me in this issue.. i got stuck here from long time.