Listbox validation problem in Javascript and asp.net

I got this custom validator which I want to check whether this left listbox is empty and ask user to add items from the 2nd right listbox next to it. However, I got an javascript error when I put this custom validator in my aspx.  If I take away the validator from html, adding items works.

if the validator exists in the html aspx,  adding items does not work. Please help.

   <asp:CustomValidator ID="cvChosenList" runat="server" Display="Dynamic" ErrorMessage="Managed By is required" ClientValidationFunction="assignedAreaCheck"  Width="156px">Managed By is required.</asp:CustomValidator>

and javascript


<script language="JavaScript" type="text/javascript">

function assignedAreaCheck(sender, args)
{
   var listBox = document.getElementById('lstChosenManaged');
   
   if (listBox.length>0)
   args.IsValid = true;
   else
   args.IsValid = false;

}
            
</script>

spiralAsked:
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Erick37Commented:
What is the script error you are getting?

Maybe use ClientID to properly reference the Listbox:

var listBox = document.getElementById('<%=lstChosenManaged.ClientID%>');
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spiralAuthor Commented:
thanks Erick37.

I got an error object not found. After changing the js function, the error disappears.

I also need to change the causevalidation property of my add button to false so the validation logic will not kick in when I add items.
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