title: AVUser, AVFile reports error
body: <pre><code>import UIKit
  
  class SignUpVC: UIViewController {
  @IBOutlet weak var usernameTxt: UITextField!
  ...
  @IBOutlet weak var cancelBtn: UIButton!
  
  @IBAction func signUpBtn_clicked(_ sender: AnyObject) {
  
      let user = AVUser()
      user.username = usernameTxt.text?.lowercased()
      user.email = emailTxt.text?.lowercased()
      user.password = passwordTxt.text
      user["fullname"] = fullnameTxt.text?.lowercased()
      user["bio"] = bioTxt.text
      user["web"] = webTxt.text?.lowercased()
      user["gender"] = ""
  
  
      user.signUpInBackground{ (success: Bool, error:Error?) in
          if success{
              print("Signup OK")
          } else {
              print(error?.localizedDescription)
          }
      }
  </code></pre>
  
  <p>I am trying to write an app just like Instagram. I wanna send the signup information to leancloud account. Why it always report error in the following line:</p>
  
  <pre><code>let user = AVUser()
  </code></pre>
  
  <p>Thanks for your help !</p>