title: ''Value of type 'AuthDataResult?' has no member 'uid'''
body: <pre><code>@IBAction func SignUpBtn_TouchUpInside(_ sender: Any) {
  
  Auth.auth().createUser(withEmail: emailTextField.text!, password: passwordTextField.text!, completion:  { (user: AuthDataResult?, error:Error?) in
      if error != nil{
          print(error?.localizedDescription)
          return
  
  }
  
  let ref = Database.database().reference()
  let usersRenfence = ref.child("users")
  
  let uid = user.uid 
  //(HERE IS WRONG, I DONT KNOW WHAT SHOULD I DO. It's my first time study code.)
  let newUserRefernce = ref.child(uid)
  newUserRefernce.setValue{["username": self.usernameTextField.text!, "email": self.emailTextField.text!]
  
  print("description: \(newUserRefernce.description())")
  </code></pre>