[9.0][FIX][mail_tracking] Control exception when mail_message was rem…

…oved (OCA#132)

mail_tracking/models/mail_tracking_email.py

@@ -225,6 +225,8 @@ def tracking_img_add(self, email):
         return email
 
     def _message_partners_check(self, message, message_id):
+        if not self.mail_message_id.exists():  # pragma: no cover
+            return True
         mail_message = self.mail_message_id
         partners = (
             mail_message.needaction_partner_ids | mail_message.partner_ids)