System.NullReferenceException: 'Object reference not set to an instance of an object.' bilgiislemm.Models.Account.RidNavigation.get returned null

        private IEnumerable<Claim> getUserClaims(Account account)
    {
        List<Claim> claims = new List<Claim>();
        claims.Add(new Claim(ClaimTypes.Name, account.Pos));
        claims.Add(new Claim(ClaimTypes.Role, account.RidNavigation.Name)); //RidNavigationdu bozmak istemedim.
        return claims;
    }

I am trying to login but System.NullReferenceException: 'Object reference not set to an instance of an object.'

bilgiislemm.Models.Account.RidNavigation.get returned null. I am encountering the error. My codes in login controller are as follows

        [HttpPost]
    public IActionResult Index(Account p)
    {
        var account = db.Accounts.SingleOrDefault(a => a.Pos == p.Pos && a.Sif == p.Sif);
        if (account != null)
        {
            SecurityManager securityManager = new SecurityManager();
            securityManager.SignIn(this.HttpContext, account);
            return RedirectToAction("Index", "Home");
        }
        else
        {
            ViewBag.error = "Hatalı";
            return View();
        }
    }

When I run the application for the first time, when I try to log in with the correct information, I can log in. But after trying to enter with wrong information, when I try to enter with correct information again, I encounter this error.