[FIX] Event: trace back on group by search

I have resolve the trace back that comes when trying to group by on the events.

More Details:
Task: https://www.odoo.com/web#id=9007&view_type=form&model=project.task&menu_id=3942&action=327

Pad: https://pad.odoo.com/p/r.oN0lvbKmiFUpDWsh

Thanks
rba-odoo