无法获取Facebook访问令牌

Question

Facebook登录无法始终正常工作。

它将始终显示登录页面，但是成功登录后，有时将无法获取访问令牌。它有时会工作，而有时却没有。这是我获取令牌的地方。

func loginButton(_ loginButton: FBSDKLoginButton!, didCompleteWith result: FBSDKLoginManagerLoginResult!, error: Error!) {
    print("Login button did complete with result")

    let accessToken = FBSDKAccessToken.current()

    guard let accessTokenString = accessToken?.tokenString else {
        ProgressHUD.showError("There was an error")
        return }

    let credentials = FacebookAuthProvider.credential(withAccessToken: accessTokenString)
    print("Granted Permissions: \(result.grantedPermissions)")

    if error != nil {
        print("There was an error")
        ProgressHUD.showError(error.localizedDescription)
        return
    } else if result.isCancelled {
        ProgressHUD.dismiss()
    } else {
        ProgressHUD.show("Loading")