Forms Data Controls :: GridView1 On First Page Fired Event PageIndexChanging Which Wasn't Handled

I am getting this error when click on page no 2 of grid view on first page it is working correctly My source code of grid view is

<asp:GridView ID="GridView1" runat="server" Width="738px" CellPadding="4" ForeColor="#333333" AllowPaging="True" >
<RowStyle BackColor="#FFFBD6" ForeColor="#333333" />
<FooterStyle BackColor="#990000" Font-Bold="True" ForeColor="White" />
<PagerStyle BackColor="#FFCC66" ForeColor="#333333" HorizontalAlign="Center" />
<SelectedRowStyle BackColor="#FFCC66" Font-Bold="True" ForeColor="Navy" />
<HeaderStyle BackColor="#990000" Font-Bold="True" ForeColor="White" />
<AlternatingRowStyle BackColor="White" />
</asp:GridView>
And c# 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;
using System.Data.SqlClient;
public partial class Time_keeping_system : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
}
private void Page_PreInit(object sender, EventArgs e)
{
if (Session["USERNAME"] == null)
{
this.MasterPageFile = "MasterPage.master";
}
else
{
this.MasterPageFile = "MasterPage2.master";
}
}
protected void Calendar1_SelectionChanged(object sender, EventArgs e)
{
//TextBox1.Text = Calendar1.SelectedDate.ToShortDateString();
TextBox1.Text = Calendar1.SelectedDate.ToString("dd/MM/yyyy");
}
protected void Button1_Click(object sender, EventArgs e)
{
if (TextBox1.Text == "*")
{
SqlCommand cmd;
SqlConnection con = new SqlConnection("Data Source=SUNTECH-812ECD7\SQLEXPRESS;Initial Catalog=suntech;Integrated Security=True;");
cmd = new SqlCommand("select ld.username, ld.login_date,ld.login_time,wp.Project_Id,wp.Project_Of,wp.Project_Name,wp.Start_time,wp.end_time, bd.break_for,bd.break_time,bd.break_over_time,l.logout_time from login_details ld inner join working_projects
wp on ld.login_date= wp.Date join break_details bd on ld.login_date = bd.login_date join logout l on bd.login_date= l.logout_date where ld.username ='" + DropDownList1.SelectedValue + "'");
//cmd = new SqlCommand("Select * from " + DropDownList3.SelectedItem + " where project_name like ''" + TextBox1.Text + "''");
//cmd.ExecuteNonQuery();
con.Open();
cmd.Connection = con;
//SqlDataReader reader = cmd.ExecuteReader();
//GridView1.DataSource = reader;
SqlDataAdapter da = new SqlDataAdapter(cmd);
DataSet ds = new DataSet();
da.Fill(ds);
GridView1.DataSource = ds;
GridView1.DataBind();
con.Close();
// reader.Close();
}
else
{
SqlCommand cmd;
SqlConnection con = new SqlConnection("Data Source=SUNTECH-812ECD7\SQLEXPRESS;Initial Catalog=suntech;Integrated Security=True;");
cmd = new SqlCommand("select ld.username, ld.login_date,ld.login_time,wp.Project_Id,wp.Project_Of,wp.Project_Name,wp.Start_time,wp.end_time, bd.break_for,bd.break_time,bd.break_over_time,l.logout_time from login_details ld inner join working_projects
wp on ld.login_date= wp.Date join break_details bd on ld.login_date = bd.login_date join logout l on bd.login_date= l.logout_date where ld.username = '" + DropDownList1.SelectedValue + "' AND ld.login_date = '" + TextBox1.Text + "'");
//cmd = new SqlCommand("Select * from " + DropDownList3.SelectedItem + " where project_name like ''" + TextBox1.Text + "''");
//cmd.ExecuteNonQuery();
con.Open();
cmd.Connection = con;
//SqlDataReader reader = cmd.ExecuteReader();
//GridView1.DataSource = reader;
SqlDataAdapter da = new SqlDataAdapter(cmd);
DataSet ds = new DataSet();
da.Fill(ds);
GridView1.DataSource = ds;
GridView1.DataBind();
con.Close();
// reader.Close();
}
}
protected void TextBox1_TextChanged(object sender, EventArgs e)
{
}
}