Excuting Connectins In Asp.Net I Am Getting Error, I Am Getting Null Reference Exception When Excuting The Codi

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

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

public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            BindGridView();
        }
    }

    protected void BindGridView()
    {
        DataTable dt = new DataTable();
        SqlDataAdapter da = new SqlDataAdapter("Select eid,ename,age from emp", con);
        con.Open();
        da.Fill(dt);
        con.Close();

        if (dt.Rows.Count > 0)
        {
            GridView1.DataSource = dt;
            GridView1.DataBind();
        }
    }
    
    protected void GridView1_SelectedIndexChanging(object sender, GridViewSelectEventArgs e)
    {
        TextBox txt_name = new TextBox();
        TextBox txt_age = new TextBox();

        TextBox txt_id = new TextBox();
        txt_id = (TextBox)GridView1.FooterRow.FindControl("txt_eid");
        txt_name = (TextBox)GridView1.FooterRow.FindControl("txt_ename");
        txt_age = (TextBox)GridView1.FooterRow.FindControl("txt_eage");

        SqlCommand cmd = new SqlCommand("insert into emp(eid,ename,age)values('" + txt_id.Text + "','" + txt_name.Text + "','" + txt_age.Text + "')");

        con.Open();
        cmd.ExecuteNonQuery();
        con.Close();
        BindGridView();
    }
        
}