User Control Disappears After PostBack

//default page
 
using System;
using System.Configuration;
using System.Data;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
 
public partial class _Default : System.Web.UI.Page 
{
    protected void Page_Load(object sender, EventArgs e)
    {
    }
    private void LoadMain(string controle)
    {
        UserControl uc = (UserControl)LoadControl(controle);
        Session["control"] = uc;
        main.Controls.Clear();
        main.Controls.Add(uc);
    }
    protected void bt_c1_Click(object sender, EventArgs e)
    {
        LoadMain("c1.ascx");
    }
    protected void bt_c2_Click(object sender, EventArgs e)
    {
        LoadMain("c2.ascx");
    }
}