无法从 Swift 中的 Facebook iOS SDK 捕获城市和国家/地区值

我目前正在使用 Facebook iOS SDK 在我的应用程序中成功捕获电子邮件、名字和姓氏。但是，由于某种原因，我很难为用户捕获位置信息，例如城市和国家/地区。

这是我的相关代码：

override func viewDidLoad() {

        super.viewDidLoad()
        fbLoginButton.delegate = self
        fbLoginButton.readPermissions = ["email", "public_profile", "user_location"]
}

extension RegisterViewController: FBSDKLoginButtonDelegate {

    func loginButtonDidLogOut(_ loginButton: FBSDKLoginButton!) {
        print("Did log out of facebook")
    }

    func loginButton(_ loginButton: FBSDKLoginButton!, didCompleteWith result: FBSDKLoginManagerLoginResult!, error: Error!) {
        if error != nil {
            print(error)
            return
        }

        print("Successfully logged in with facebook...")
        FBSDKGraphRequest(graphPath: "/me", parameters: ["fields": "id, name, first_name, last_name, email, location{location}"]).start { (connection, result, err) in

            if err != nil {
                print("Failed to start graph request:", err)
                return
            }

            print(result)

            let fbDetails = result as! NSDictionary
            let location : NSDictionary! = fbDetails.value(forKey: "location") as! NSDictionary //code crashes here
            let locationContents : NSDictionary! = location.value(forKey: "location") as! NSDictionary
            let city: String! = locationContents.value(forKey: "city") as? String
            let state: String! = locationContents.value(forKey: "state") as? String
            let country: String! = locationContents.value(forKey: "country") as? String

            print(fbDetails)
            print("city:", city)
            print("state:", state)
            print("country:", country)


        }
    }
}

当我运行代码时，在我的控制台中，我看到了电子邮件、名字、姓氏、ID、姓名的输出，但没有任何位置信息，我的应用程序崩溃了：

fatal error: unexpectedly found nil while unwrapping an Optional value 

当它到达线时：

let location : NSDictionary! = fbDetails.value(forKey: "location") as! NSDictionary

谁能看到我做错了什么？