Object reference not set to an instance of an object. error

s sir, but i am new to session creation, please help me how to clear this..?

this is my code...

my web form code....

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Data.SqlClient;
using System.Configuration;
using System.Data;

public partial class menu_CreateMenu : System.Web.UI.Page
{
public string connection = ConfigurationManager.ConnectionStrings["aspnetdbConnectionString"].ToString();
string choice = "0";

protected void Page_Load(object sender, EventArgs e)
{
SqlConnection connection = new SqlConnection("Data Source=(local);Initial Catalog=aspnetdb;User ID=sa;Password=voyage");

if (!IsPostBack)
{
FillddlMainMenu();
choice = Session["Choice"].ToString();
if (choice == "Main")
{
pnlMainMenu.Visible = true;
}
else if (choice == "Sub")
{
pnlSubMenu.Visible = true;
}

}
else
{

}


}
protected void btnMainmenu_Click(object sender, EventArgs e)
{
int active;
if (chkMActive.Checked)
{
active = 1;
}
else
{
active = 0;
}
SqlConnection conn = new SqlConnection(connection);
string sql = "INSERT INTO MainMenu(MainMenu, MLocation, MenuOrder, IsActive)VALUES('" + txtMainmenu.Text + "','" + txtMLocation.Text + "'," + Convert.ToInt16(ddlMOrder.SelectedItem.Value) + "," + active + ")";
SqlCommand cmd = new SqlCommand(sql, conn);
conn.Open();
cmd.CommandType = CommandType.Text;
cmd.ExecuteNonQuery();
conn.Close();
FillddlMainMenu();
Response.Redirect("SuccessMsg.aspx");
}

public void FillddlMainMenu()
{
SqlConnection conn = new SqlConnection(connection);
DataSet DS = new DataSet();
SqlDataAdapter dataadapter = new SqlDataAdapter();
SqlCommand cmd = new SqlCommand("Select MainMenuId,MainMenu from MainMenu", conn);

//cmd.CommandType = CommandType.Text;
dataadapter.SelectCommand = cmd;
dataadapter.Fill(DS);
ddlMainMenu.DataSource = DS;
ddlMainMenu.DataBind();




}
protected void btnSubmenu_Click(object sender, EventArgs e)
{
int active;
if (chkSActive.Checked)
{
active = 1;
}
else
{
active = 0;
}
SqlConnection conn = new SqlConnection(connection);
string sql = "INSERT INTO SubMenu(SubMenu, SubMenuLocation, MenuOrder,ParentId, IsActive)VALUES('" + txtSubmenu.Text + "','" + txtSubMenuLocation.Text + "'," + Convert.ToInt16(ddlSOrder.SelectedItem.Value) + "," + Convert.ToInt16(ddlMainMenu.SelectedItem.Value) + "," + active + ")";
SqlCommand cmd = new SqlCommand(sql, conn);
conn.Open();
cmd.CommandType = CommandType.Text;
cmd.ExecuteNonQuery();
conn.Close();
Response.Redirect("SuccessMsg.aspx");
}
}



master page code is:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Data.SqlClient;
using System.Data;
using System.Configuration;

public partial class menu_MasterPage : System.Web.UI.MasterPage
{
static DataSet static_dataset = new DataSet();

DataTable dt = new DataTable();
public string connection = ConfigurationManager.ConnectionStrings["aspnetdbConnectionString"].ToString();

protected void Page_Load(object sender, EventArgs e)
{
//if (!IsPostBack)
//{
static_dataset = ds();
dt = static_dataset.Tables[0].Copy();
dt.Clear();