ASP.NET MVC get userId when creating user?

I want to get UserId when registering

[AllowAnonymous]
    [HttpPost]
    public ActionResult Register(RegisterModel model)
    {
        if (ModelState.IsValid)
        {
            // Attempt to register the user
            MembershipCreateStatus createStatus;
            Membership.CreateUser(model.UserName, model.Password, model.Email, passwordQuestion: null, passwordAnswer: null, isApproved: true, providerUserKey: null, status: out createStatus);

            if (createStatus == MembershipCreateStatus.Success)
            {
                FormsAuthentication.SetAuthCookie(model.UserName, createPersistentCookie: false);
                //Object reference not set to an instance of an object.
                Guid id = (Guid)System.Web.Security.Membership.GetUser().ProviderUserKey;
                System.Web.Security.Roles.AddUserToRole(model.UserName, "User");
                return RedirectToAction("Index", "Home");
            }
            else
            {
                ModelState.AddModelError("", ErrorCodeToString(createStatus));
            }
        }

        // If we got this far, something failed, redisplay form
        return View(model);
    }

But an error occurred ("The reference to the object is not installed in the instance of the object."). How to get userId method in register?

Thank.