how to rectify error " grid view fired event PageIndexChanging which wasn't handled."

protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
{
                   SqlConnection conn = new SqlConnection(str);
                   SqlCommand cmd = new SqlCommand("spshow",conn);
                   cmd.CommandType = CommandType.StoredProcedure;
                   cmd.Parameters.AddWithValue("@var", ddlshow.SelectedValue);
                   SqlDataAdapter adp = new SqlDataAdapter(cmd);
                   DataSet ds = new DataSet();
                   adp.Fill(ds);

                   try
                   {
                       conn.Open();
                       gdshow.Visible = true;
                       gdshow.DataSource = ds;
                       gdshow.DataBind();
                   }

                   catch (Exception E)
                   {
                       Response.Write(E.Message);

                   }
                   finally
                   {
                       conn.Close();

                   }
}