balloonpopupextender inside datalist

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ASP.NET

Ajax

datalist

I was planning to use ballonpopupextender to popup after value input in each textbox generated in datalist. this popup must appear once a new number is entered in textbox and it will contain the total (SUM) of all textbox. I wrote the below code but I'm getting the error.

0x800a139e - JavaScript runtime error: Referred BalloonPopupControlId did not find.

<asp:DataList ID="objDatalist" runat="server" DataSourceID="DSObjectives" Width="100%">
    <ItemTemplate>
        <asp:Panel ID="PnlSum" runat="server" CssClass="FixedLabel" Visible="False">
            <asp:Label ID="SUMLabel" runat="server" CssClass="FullWidth" Text="Total Weights"></asp:Label><br />
            <asp:Label ID="lblSUM" runat="server" CssClass="FullWidth" Text="0%"  Font-Bold="True" Font-Size="X-Large" ForeColor="#F47723" ></asp:Label>
        </asp:Panel>
        <ajaxToolkit:BalloonPopupExtender ID="BalloonPopupExtender1"  runat="server"
            TargetControlID="txtWeight" BalloonPopupControlID="PnlSum"
            DisplayOnFocus="false" DisplayOnClick="true" Position="TopRight"
            UseShadow="true" BalloonSize="Medium" BalloonStyle="Cloud" >
        </ajaxToolkit:BalloonPopupExtender>
        <asp:HiddenField ID="ObjectiveID" runat="server" Value='<%# Eval("ObjectiveID")%>' />
        <table style="width: 100%;">
            <tr style="width: 100%; border:1px solid #555555; background-color:#f0f0f0; ">
                <td style="width: 70%; padding:3px; padding-removed30px">
                    <asp:TextBox ID="txtObjective" runat="server" Width="99%" TextMode="MultiLine" Height="38px" Text='<%# Eval("ObjectiveName")%>'></asp:TextBox>
                </td>
                <td style="width: 12%; padding:1px; font-weight:bold;">
                    <asp:TextBox ID="txtweight" runat="server" Width="95%" Text='<%# Eval("ObjectiveWeight")%>' OnTextChanged="txtweight_TextChanged" AutoPostBack="True"></asp:TextBox>
                </td>
            </tr>
            <tr style="width: 100%;height:5px;"></tr>
        </table>

    </ItemTemplate>
</asp:DataList>