Can't get value (selected value) from popup select component

hi, im using popup select to retrieve searched value. i cant use multi select(select2) component because of very long list of selection. problem here when using popup select, it's return no value. try to test selected value using jquery, null value returned. i'm also did test using multiselect, the value (id) is returned. 

<script>
$(document).ready(function(){
var select = FormUtil.getField("numFilePicker");


$(select).on("change", function(){
var value = FormUtil.getValue("popupSelectBox");
alert("selected value : " + value);
});
});
</script>