Enterprise portal- adding a tunnel in page

Dynamics 365 for Finance and Operations/AX Technical Forum
#1 
using System;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using Microsoft.Dynamics.Framework.Portal.UI.WebControls;
using Microsoft.Dynamics.Framework.Portal.UI.WebControls.WebParts;

public partial class CarRentalCreateTunnel : System.Web.UI.UserControl
{	
    protected void Page_Load(object sender, EventArgs e)
    {
    }
    protected void WizardTunnel_FinishButtonClick(object sender,WizardNavigationEventArgs e)
    {
        
        AxDataSourceView rentalTableView = this.RentalDataSet.GetDataSourceView("RentalTable");
        if (rentalTableView != null)
            rentalTableView.EndEdit();
        AxUrlMenuItem carRentalsMenuItem =
        new AxUrlMenuItem("CarRentalList");
        Response.Redirect(carRentalsMenuItem.Url.ToString());
    }
}

I’ve added dataset as rental table in my design part for my control

I added a button and when I try using this coding I’m getting an error

I’m trying this example from ax-2009 programming getting started book.

May I know why I’m getting this error?

Thanks