[FIX] mail: fix mailing wizard when partners and template specified

if partner_ids and template_id are set, a traceback happens.

Indeed, some mail fields are not taken from the res_ids when partner_ids
is set.

When trying to update the values with the one from the template, the
existing values are popped, which raised an error as those keys weren't
set.

closes odoo/odoo#38298

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

addons/mail/wizard/mail_compose_message.py

@@ -474,9 +474,9 @@ class MailComposer(models.TransientModel):
         for res_id in res_ids:
             if template_values.get(res_id):
                 # recipients are managed by the template
-                results[res_id].pop('partner_ids')
-                results[res_id].pop('email_to')
-                results[res_id].pop('email_cc')
+                results[res_id].pop('partner_ids', None)
+                results[res_id].pop('email_to', None)
+                results[res_id].pop('email_cc', None)
                 # remove attachments from template values as they should not be rendered
                 template_values[res_id].pop('attachment_ids', None)
             else: