System.NullReferenceException error Rate this

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
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)
        {
            SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString);
            conn.Open();
            string checkuser = "select count (*) from UsersData where UserName= '" + UserName.Text + "'";
            SqlCommand com = new SqlCommand(checkuser, conn);
            int temp = Convert.ToInt32(com.ExecuteScalar().ToString());
            if (temp == 1)
            {
                Response.Write("User Already Exists");
            }
            conn.Close();
        }
    }
    
    protected void Submit_Click(object sender, EventArgs e)
    {
        try
        {
            Guid newGUDI = Guid.NewGuid();

            SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["UsersConnectionString"].ConnectionString);
            conn.Open(); //line 35?
            string insertQuery = "insert into UsersData (ID, UserName, Password, CellPhone, DPT) values (@ID ,@Uname ,@Pass ,@Cell ,@DPT)";
            SqlCommand com = new SqlCommand(insertQuery, conn);
            com.Parameters.AddWithValue("@ID", newGUDI.ToString());
            com.Parameters.AddWithValue("@Uname", UserName.Text);
            com.Parameters.AddWithValue("@Pass", Password.Text);
            com.Parameters.AddWithValue("@DPT", DPT.SelectedItem.Text);

            com.ExecuteNonQuery();
            Response.Redirect("UsersManger.aspx");
            Response.Write("Registration SUCESSFULL");

            conn.Close();
        }
        catch (Exception ex)
        {
            Response.Write("Error:" + ex.ToString());
        }
    }
    protected void DPT_SelectedIndexChanged(object sender, EventArgs e)
    {

    }
}