there is a fatal error what it means and how to solve i try to get text from textfield in swift 3

Question

class Logincontroller: UIViewController
{
    @IBOutlet weak var login_tf_uname: UITextField!

    @IBOutlet weak var login_tf_pwd: UITextField!

    override func viewDidLoad()
    {
        super.viewDidLoad()

    }

    override func didReceiveMemoryWarning()
    {
        super.didReceiveMemoryWarning()
    }


    @IBAction func submit(_ sender: Any)
    {
        let uid = login_tf_uname.text!
        print(uid)
        let pwd = login_tf_pwd.text!
        print(pwd)
    }

}