Ribbon Workbench Display Rule with JavaScript

Hello. Since xrm.page has been depricated I try to move over to using ExecutionContext instead but I run into problem.

In the display rule I have added a crm parameter with primaryControl

The javascript fails on:  var formContext = parentControl.getFormContext();   

And the beginning of the javascript looks like:

function IsCurrentUserOwner(parentControl)

{

  var formContext = parentControl.getFormContext();   

  var owner= formContext.getAttribute("ownerid").getValue();

  var ownerID = owner[0].id;

  var userId = Xrm.Page.context.getUserId();

  if (ownerID == userId ) {

     return true;

  } else { 

                return false;

  }

}

 Can someone point out what I have missed?