how to do page load issues ,button Click Issues

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ASP.NET4

Add button click popup window comes then hit the cancel button from Pop window . after that . Click the Add button Again its not taking single click . it taking double click. why .what the region behind it .
Aspx.cs
this is for AddButton

protected void btnAddNewGroup_Click(object sender, EventArgs e)
        {

            Get_Users_List(gv_Users);
            lbl_ModalPopupExtender1_msg.Text = "";

            ModalPopupExtender1.Show();
            txt_GroupName.Focus();
        }

this is for cancel button

protected void Btn_Cancel_Click1(object sender, EventArgs e)
        {
            div_All.Visible = true;
            div_Group.Visible = false;
            L_Message.Visible = false;

            //   Populate_Groups_PageWise(userid, "");
            Populate_Groups(userid);
            MenuItem item = NavigationMenu.FindItem("All");
            if (item != null)
            {
                item.Selected = true;
            }
        }