NullReferenceException

Question

下面的代码的“尝试捕获”块中有问题。 在函数SetUser中，如果用户存在于数据库中，则使用getId函数返回一个ID，否则，我将收到NullReferenceException。
我在Login中的try catch块中调用此函数。我有一个问题，因为当生成异常时，我希望将用户重定向到注册页面。但是，当我尝试与不存在的用户执行代码时，我认为我有一种无限循环，因为我的页面不会停止加载。我不明白我在做什么错。需要帮助

功能登录：

 public static void Login(HttpRequest Request, HttpResponse Response, string redirectUri)
{
    if (Request.IsAuthenticated)
        return;
    if (!Request.Form.AllKeys.Contains("id_token"))
        return;
    string value = Request.Form.Get("id_token");
    JObject id_token = JwtDecode(value);
    string upn = id_token.GetValue("upn").ToString();
    DateTime expiretime = GetExpireTime(id_token);

    try
    {
        SetUser(id_token);
    }
    catch (Exception ex)
    {
        Response.Redirect("~/register.aspx");
    }
    FormsAuthenticationTicket ticket = new FormsAuthenticationTicket(1, upn, DateTime.UtcNow, expiretime, false, id_token.ToString(), FormsAuthentication.FormsCookiePath);
    string encryptedcookie = FormsAuthentication.Encrypt(ticket);
    HttpCookie cookie = new HttpCookie(FormsAuthentication.FormsCookieName, encryptedcookie);
    cookie.Expires = expiretime;
    Response.Cookies.Add(cookie);
    redirectUri = GetRedirectUrl(Request, redirectUri);
    Response.Redirect(redirectUri, true);
}

功能setUser：

 private static void SetUser(JObject id_token)
{
    string email = id_token.GetValue("unique_name").ToString();
    string name = id_token.GetValue("given_name").ToString();
    DataSet ds;

    List<Claim> claims = new List<Claim>()
    {
        new Claim(ClaimTypes.Email, email),
        new Claim(ClaimTypes.Name, GetId(email))
    };

    string roles= "SELECT name FROM AspNetRoles;
    ds = GetDataSet(roles);

    if (ds.Tables.Count > 0)
    {
        foreach (var row in ds.Tables(0).Rows)
            claims.Add(new Claim(ClaimTypes.Role, row("name")));
    }

    ClaimsIdentity claimsIdentity = new ClaimsIdentity(claims, "Cookies");
    ClaimsPrincipal principal = new ClaimsPrincipal(claimsIdentity);
    HttpContext.Current.GetOwinContext().Authentication.User = principal;
    Thread.CurrentPrincipal = principal;
}

function getId：

 public static string getId(string email)
{
    return ((new UserManager()).FindByEmail(email)).Id;
}