error is >>> system.ui.is null

I am trying to get rid of an error comming from the ajax control toolkit the error is >>> system.ui.is null

I tired everything I can think of inculding deleting and re adding the text box and ajaxcontrol, making it null ?

when in debug mode I see this error below

$create(Sys.Extended.UI.DropDownBehavior, {"dynamicServicePath":"/WebApps/QA/input.aspx","id":"txt_LISTING_DATE_DropDownExtende"}, null, null, $get("txt_LISTING_DAT"));