When this method is called without an event (a.k.a. `event=None`), this method produces the following error:
```
Traceback (most recent call last):
File "/opt/odoo/auto/addons/mail/models/mail_mail.py", line 278, in send
res = IrMailServer.send_email(msg, mail_server_id=mail.mail_server_id.id)
File "/opt/odoo/auto/addons/mail_tracking/models/ir_mail_server.py", line 88, in send_email
tracking_email.smtp_error(self, smtp_server_used, e)
File "/opt/odoo/auto/addons/mail_tracking/models/mail_tracking_email.py", line 213, in smtp_error
self.sudo()._partners_email_bounced_set('error')
File "/opt/odoo/auto/addons/mail_tracking/models/mail_tracking_email.py", line 203, in
_partners_email_bounced_set
]).email_bounced_set(self, reason, event=event)
File "/opt/odoo/auto/addons/mail_tracking_mailgun/models/res_partner.py", line 22, in
email_bounced_set
self._email_bounced_set(reason, event)
File "/opt/odoo/auto/addons/mail_tracking_mailgun/models/res_partner.py", line 33, in
_email_bounced_set
event['Message-Id'] or '') TypeError: 'NoneType' object has no attribute '__getitem__'
```
So, we now assume we do not always have an event.
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