Gettings IDs shown in a subgrid - CRM 2016?

Hi

In CRM 2013 it was possible to get the ids of items shown in a subgrid via code such as:

var selectedItems = document.getElementById("cases").control.get_selectedIds();

In CRM 2016 this returns null, presumably due to the document.getElementByID not being supported. Is there a way this can be achieved in 2016?

I tried:

var selectedItems = Xrm.Page.getControl("cases").get_selectedIds();

But that also returned null.