Add fetchXml filter to a subgrid in CRM 2013

Hi all,

I'm trying to add a filter fetchXml in a subgrid view in CRM 2013 as I did in the 2011 version but when I'm trying to get the control of the subgrid it returns me a null value.

I'm trying the following commands:

-Xrm.Page.getControl('accountactivities_d')

-Xrm.Page.getControl('accountactivities')

-document.getElementById('accountactivities_d').control.SetParameter(“fetchXml”, fetchXml);

-document.getElementById('accountactivities_d').SetParameter(“fetchXml”, fetchXml);

-document.getElementById('accountactivities_d').control.Refresh();

Does anybody know how can I do to take the control, and then add the filter and refresh the grid?

Many thanks to all!