Call JavaScript function after UpdatePanel Refresh (Partial PostBack) is completed in ASP.Net

Hi

http://www.aspsnippets.com/Articles/Filter-and-Search-ASP.Net-DropDownList-items-using-JavaScript.aspx I am using this functionality in my project and it is working very fine but i am facing very strange issue while using this.

The above functionality is not working when we place dropdownlist and textbox inside updatePanel and ModalPopupExtender

The .aspx page where i am using ModalpopupExtender and UpdatePanel for DropDownList and Textbox is below

        <%-- ***************************  MODAL POPUP EXTENDER ***************************************************** --%>
    <asp:HiddenField ID="HiddenField3" runat="server" />
    <asp:HiddenField ID="HiddenField4" runat="server" />

    <cc1:ModalPopupExtender ID="ModalPopupExtender1" runat="server" PopupControlID="Div6" TargetControlID="HiddenField3"
                            BackgroundCssClass="modalBackground" CancelControlID="bclosemodalpopup" >
    </cc1:ModalPopupExtender>
            <%-- ***************************  MODAL POPUP ***************************************************** --%>
<div ID="Div6" runat="server" class="modalPopup" align="center" style = "display:none; overflow:scroll;">
        <br />
<asp:Button ID="button" runat="server"
        OnClick="button_Click" style="display:none;"/>
<asp:UpdatePanel ID="UpdatePanel1" ChildrenAsTriggers = "true"  UpdateMode="conditional" runat ="server"  >
<ContentTemplate>

        <asp:TextBox ID="TextBox1" runat="server" onkeyup = "FilterItems(this.value)" CausesValidation="true"></asp:TextBox><br />
        <asp:DropDownList Width="150px"  ID="DropDownList1" runat="server" 
            DataSourceID="sdcEnglishAuthor" DataTextField="AuthorName" DataValueField="id"/>
            <%-- <cc1:ListSearchExtender ID="ListSearchExtender2" runat="server" TargetControlID="DropDownList1"/> 
            --%>
        <asp:Label ID = "label12222" runat="server"></asp:Label>

        <asp:Button ID="bclosemodalpopup" runat="server" Text="close" />
</ContentTemplate>
 <Triggers>
     <asp:AsyncPostBackTrigger ControlID="button" EventName="Click" />
   </Triggers>
   </asp:UpdatePanel>
</div>

The above code is not working with UpdatePanel but it is working if we remove DropdownList and Textbox from updatepanel.

The javascript code which i am using is below

<script type = "text/javascript">
 
    var ddlText, ddlValue, ddl, lblMesg;
    function CacheItems() {
        ddlText = new Array();
        ddlValue = new Array();
        ddl = document.getElementById("<%=DropDownList1.ClientID %>");
        lblMesg = document.getElementById("<%=label12222.ClientID%>");
        for (var i = 0; i < ddl.options.length; i++) {
            ddlText[ddlText.length] = ddl.options[i].text;
            ddlValue[ddlValue.length] = ddl.options[i].value;
        }
    }
    window.onload = CacheItems;

    function FilterItems(value) {
        //alert("hello");
        var button = document.getElementById('<%= button.ClientID %>');
        ddl.options.length = 0;
        for (var i = 0; i < ddlText.length; i++) {
            if (ddlText[i].toLowerCase().indexOf(value) != -1) {
                AddItem(ddlText[i], ddlValue[i]);
                
            }
        }
        lblMesg.innerHTML = ddl.options.length + " items found.";
        if (ddl.options.length == 0) {
            AddItem("No items found.", "");
        }
        
        
        //__doPostBack("<%=UpdatePanel1.ClientID %>", "");
        //__doPostBack("<%= button.ClientID %>", "");
    }

    function AddItem(text, value) {
        var opt = document.createElement("option");
        opt.text = text;
        opt.value = value;
        ddl.options.add(opt);
    }
</script>

Please help on this