[FIX] fix helper

mail_usability/models/mail_message.py

@@ -14,5 +14,7 @@ class MailMessage(models.Model):
         # we do not use a reference field here as mail message
         # are used everywhere and many model are not yet loaded
         # so odoo raise exception
+        if self:
             self.ensure_one()
             return self.env[self.model].browse(self.res_id)
+        return None