Browser : Internet Explorer 9

ASP.Net Type : ASP.Net 4.0 WebForms

 

Based on the example (Start-uploading-manually.aspx), I  had copied the following two JavaScript functions in a seperate Javascript file (FileUpload.js):

function submitbutton_click() 

function CuteWebUI_AjaxUploader_OnPostback()

I had referred the above Javascript file in my "Default.aspx" web form

 <asp:ScriptManager ID="ScriptManager1" runat="server">                    
<Scripts>                    
<asp:ScriptReference Path="~/Scripts/FileUpload.js" />                    
</Scripts>               
</asp:ScriptManager>

Ocasssionaly, i get the following error:
SCRIPT5007: Unable to get value of the property 'appendChild': object is null or undefined.

How to resolve this error?