[FIX]Event: trace back on group by search

Hello,

I have resolved the tracebaack 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