FCK Editor with Modal PopUp Extender

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ASP.NET

I am showing FCK Editor in Modal Modal PopUp Extender but when page load it's throwing a JS Error, i.e.
htmlfile: Could not complete the operation due to error 800a025e

and here is my simple code. This code working fine on Button Click .

<asp:Button ID="btnAddSections" runat="server" Text="Add NIT/RFP" />

<asp:ModalPopupExtender ID="ModalPopupExtender1" runat="server" CancelControlID="btnCancel"
OkControlID="btnOk" TargetControlID="btnAddSections" PopupControlID="Panel1"
Drag="false" BackgroundCssClass="ModalPopupBG">
</asp:ModalPopupExtender>

<asp:Panel ID="Panel1" runat="server" CssClass="Popup">
<FCKeditorV2:FCKeditor ID="txtMesg" BasePath="~/res/fckeditor/" runat="server" Height="400px"
Width="100%" />
</asp:Panel

i havn't got any Solution .....

Thanks in advance.....

Best Regards,
Sutotpal