I got a error in my project if there some please give the solution. I have post my coding with this and error.

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


public partial class regg : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        Label3.Text = "Mark Attendance for ";
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
       
    }
    protected void btnsaveattedance_Click(object sender, EventArgs e)
    {
        int rnum = 0;
        foreach (GridViewRow row in GridView1.Rows)
        {

            int rollno = Convert.ToInt32(row.Cells[0].Text);
            String nameofthestudent = row.Cells[1].Text;
            RadioButton rbtn1 = (RadioButton)GridView1.Rows[rnum].Cells[12].FindControl("rbtnpresent1");[the error is placed there that ArgunmanetOutOfReference was unhandle by user code]

            RadioButton rbtn2 = (RadioButton)GridView1.Rows[rnum].Cells[12].FindControl("rbtnabsent");


            String status1;
            if (rbtn1.Checked)
            {
                status1 = "Present";

            }
            else
            {
                status1 = "Absent";
            }
            String dateofclass = DateTime.Now.ToShortDateString();
            String batch = DropDownList1.SelectedItem.Text;
            String semester = DropDownList2.SelectedItem.Text;
            String subjectcode = DropDownList3.SelectedItem.Text;
            String subjectname = DropDownList4.SelectedItem.Text;
            String period = DropDownList5.SelectedItem.Text;
            String date = DropDownList6.SelectedItem.Text;
            String month = DropDownList7.SelectedItem.Text;
            String year = DropDownList8.SelectedItem.Text;

            savestudenttt(rollno, nameofthestudent, dateofclass, batch, semester, status1, subjectcode, subjectname, period,date,month ,year);
            rnum++;
        }
        Label4.Text = "Attendance Has Been Saved Successfully";

    }

    private void savestudenttt(int rollno, string nameofthestudent, string dateofclass, string batch, string semester, string status1, string subjectcode, string subjectname, string period, string date, string month, string year)
    {
        String query = "insert into Attedancestatus(rollno,nameofthestudent,dateofclass,batch,semester,subjectname,subjectcode,period,date,month,year)values  (" + rollno1 + ",'" + nameofthestudent + "','" + dateofclass + "','" + status1 + "','" + batch + "','" + semester + "','" + subjectcode + "','" + subjectname + "')";
        String mycon = "Data Source=DESKTOP-15TQB1B\\SQLEXPRESS;Initial Catalog=studenttt;Integrated Security=True";
        SqlConnection con = new SqlConnection(mycon);
        con.Open();
        SqlCommand cmd = new SqlCommand();
        cmd.CommandText = query;
        cmd.Connection = con;
        cmd.ExecuteNonQuery();
    }



    public string rollno1 { get; set; }
}