迅速致命的错误

Question

当我点击退出按钮时，我想回到mainviewcontroller，但它给了我：

  

致命错误：在解包可选值时意外发现nil

navigationItem.leftBarButtonItem = UIBarButtonItem(title: "Logout", style: .plain, target: self, action: #selector(logOut))


 func logOut(){

    let mainViewController = MainViewController()
    present(mainViewController, animated: true, completion: nil)
}