Conditions:

• the custom control is used in XPiNC
• newValue property is set to false

Manifestation: You can see error "c is null on line..." in status bar

Solution: Change script in onChange event of OK button to:

var d=dijit.byId('#{javascript:compositeData.dialogID}');
var c=document.getElementById("new#{javascript:compositeData.dialogID}");
var f=document.getElementById("#{javascript:getClientId(compositeData.inputBinding)}");
f.value=d.getSelectedValues();
d.hide();
if(c!=null){
 if(c.value!="" && c.value!=null){
  d.addItem(c.value);
  d.sort();
  d.refreshList();
  c.value="";
 }
}