[FIX] mail: do not reset activity type with wrong model

Just warn models are invalid, but do not reset for stable. Data is not enforced
hence no real issue with wrong models.

Followup of odoo#156731

closes odoo#218549

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>

Author: tde-banana-odoo

addons/mail/models/mail_activity_mixin.py

@@ -382,7 +382,9 @@ def activity_schedule(self, act_type_xmlid='', date_deadline=None, summary='', n
                     'Invalid activity type model %s used on %s (tried with xml id %s)',
                     activity_type.res_model, self._name, act_type_xmlid or '',
                 )
-            activity_type = self._default_activity_type()
+            # TODO master: reset invalid model to default type, keep it for stable as not harmful
+            if not activity_type:
+                activity_type = self._default_activity_type()
 
         model_id = self.env['ir.model']._get(self._name).id
         create_vals_list = []