what is wrong in this code ? why session is not checking properly at page load event

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ASP.NET

I am always getting error

object reference not set to the instance of object


when checking if (Session["DETAILS"].ToString() == "NEW")



My source code is :

using System;
using System.Data;
using System.Configuration;
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;

public partial class SCDetails : System.Web.UI.Page
{

    Properties p = new Properties();
    Functions Fun = new Functions();

    protected void Page_Load(object sender, EventArgs e)
    {
        try
        {
            if (IsPostBack == false)
            {
                FillDD();
                if (Session["DETAILS"].ToString() == "NEW")
                {
                    Label1.Text = "NEW SUB CATEGORY DETAILS [ADD]";
                    DropDownList1.Focus();
                }
                else if (Session["DETAILS"].ToString() == "EDIT")
                {
                    if (Session["SCID"].ToString() != "")
                    {
                        Label1.Text = "EDIT SUB CATEGORY DETAILS [EDIT]";
                        Label12.Text = Session["SCID"].ToString();
                        Session["SCID"] = "";
                        GetDetails();
                        DropDownList1.Focus();
                    }
                }
            }
        }
        catch
        {

        }
    }

    protected void Button2_Click(object sender, EventArgs e)
    {
        if (Label12.Text == "")
        {
            Boolean cn;
            cn = Fun.ChkvalueExist("SELECT * FROM CMS_TB_MASTER_SUBCATEGORY WHERE CNAME='" + DropDownList1.SelectedItem.Text + "' AND SCNAME='" + TextBox1.Text + "'");
            if (cn == false)
            {
                p.CNAME = DropDownList1.SelectedItem.Text;
                p.SGNAME = TextBox1.Text;
                p.DESC = TextBox2.Text;
                string sql = "INSERT INTO CMS_TB_MASTER_SUBCATEGORY VALUES('" + p.CNAME + "','" + p.SGNAME + "','" + p.DESC + "')";
                Fun.Insert(sql);
                //fun.INFYDetails(p);
                Clear();
                Response.Write("<script language='javascript'>window.opener.document.getElementById('Button4').click();window.close();window.location='SubCategory.aspx';</script>");
            }
            else
            {
                Response.Write("<script language='javascript'>window.alert('This sub category name already exists !');</script>");
            }
        }
        else
        {
            Boolean cn, cn1;
            cn = Fun.ChkvalueExist("SELECT * FROM CMS_TB_MASTER_SUBCATEGORY WHERE SCID='" + Label12.Text + "' AND CNAME='" + DropDownList1.SelectedItem.Text + "' AND SCNAME='" + TextBox1.Text + "'");
            cn1 = Fun.ChkvalueExist("SELECT * FROM CMS_TB_MASTER_SUBCATEGORY WHERE SCID!='" + Label12.Text + "' AND CNAME='" + DropDownList1.SelectedItem.Text + "' AND SCNAME='" + TextBox1.Text + "'");
            if ((cn == true && cn1 == false) || (cn == false && cn1 == false))
            {
                p.CNAME = DropDownList1.SelectedItem.Text;
                p.SGNAME = TextBox1.Text;
                p.DESC = TextBox2.Text;
                string sql = "UPDATE CMS_TB_MASTER_SUBCATEGORY SET CNAME='" + p.CNAME + "',SCNAME='" + p.SGNAME + "', DESCP='" + p.DESC + "' WHERE SCID='" + Label12.Text + "'";
                Fun.Insert(sql);
                //fun.UDFYDetails(p);
                Clear();
                //Response.Write("<script language='javascript'>window.opener.document.getElementById('Button4').click();window.close();window.location='SubCategory.aspx';</script>");
                Response.Redirect("SubCategory.aspx");
                GetDetails();
            }
            else
            {
                Response.Write("<script language='javascript'>window.alert('This sub category name already exists !');</script>");
            }
        }
    }

    #region Functions

    public void FillDD()
    {
        string sql = "SELECT CNAME,CID FROM CMS_TB_MASTER_CATEGORY ORDER BY CNAME";
        Fun.BDD_WITH_ID(DropDownList1, sql, "--SELECT--", "CID", "CNAME");
    }

    public void GetDetails()
    {
        DataSet DS = new DataSet();
        string sql = "SELECT * FROM CMS_TB_MASTER_SUBCATEGORY WHERE SCID='" + Label12.Text + "'";
        DS = Fun.DBSelect(sql);
        if (DS.Tables[0].Rows.Count > 0)
        {
            FillDD();
            DropDownList1.SelectedValue = DS.Tables[0].Rows[0]["CNAME"].ToString();
            TextBox1.Text = DS.Tables[0].Rows[0]["SCNAME"].ToString();
            TextBox2.Text = DS.Tables[0].Rows[0]["DESCP"].ToString();
        }
        else
        {
            Clear();
            Button2.Enabled = false;
        }
    }

    public void Clear()
    {
        DropDownList1.SelectedIndex = 0;
        TextBox1.Text = "";
        TextBox2.Text = "";
    }

    #endregion
     
    protected void Button1_Click1(object sender, EventArgs e)
    {
        if (Label12.Text == "")
        {
            Clear();
        }
        else
        {
            Response.Redirect("SubCategory.aspx");
        }
    }
}