Dynamics CRM 2013 - Multi-Lookup in VS2012 Return Value Undefined


in an external project, i am trying to use the OOB Dynamics CRM 2013 Multi-Lookup window as per the below code:

var url = "[CRM_ORG_URL]/_controls/lookup/lookupmulti.aspx?class=null&objecttypes=1024&DefaultViewId=BE653221-C406-4DD1-BC80-52EC2420BDBA&DisableViewPicker=1&AllowFilterOff=0&browse=0&ShowNewButton=0&ShowPropButton=1&DefaultType=1024";
var lookupItems = window.showModalDialog(url, "entity", "dialogwidth: 35; dialogheight: 55;");

in return, lookupItems is undefined.

i am unable to retrieve back the Guids of the products selected in the Product Multi-Lookup Window (lookupItems.items)

Thank you,

Best Regards.