when i tried to use a pagination at the moment i start theproject. this error message appear " An exception of type 'System.NullReferenceException' occurred in App_Web_hqwgj1dl.dll but was not handled in user code ". Can someone help me to fix it?

my controller is : 

public class AccountController : Controller
{
    // GET: Account
    public ActionResult Index()
    {
        return View(this.GetAccounts(1));
    }

    [HttpPost]
    public ActionResult Index(int currentPageIndex)
    {
        return View(this.GetAccounts(currentPageIndex));
    }

    private AccountModel GetAccounts(int currentPage)
    {
        int maxRows = 10;
        using (GSAREntities entities = new GSAREntities())
        {
            AccountModel accountModel = new AccountModel();

            accountModel.AccountTest = (from customer in entities.AccountTest1
                                        select customer)
            .OrderBy(customer => customer.AccountName)
            .Skip((currentPage - 1) * maxRows)
            .Take(maxRows).ToList();

            double pageCount = (double)((decimal)entities.AccountTest1.Count() / Convert.ToDecimal(maxRows));
            accountModel.PageCount = (int)Math.Ceiling(pageCount);

            accountModel.CurrentPageIndex = currentPage;

            return accountModel;
        }
    }
}