TypeError: join() argument must be str or bytes, not 'NoneType'

[BojanKogoj]

If I understand this correctly it is crashing because get_language() is returning None and join doesn't accept None parameters https://github.com/peopledoc/django-mail-factory/blob/master/mail_factory/mails.py#L71

Using Django 1.11.5

Relevant traceback

/code/myapp/something/utils.py in _get_or_create_user(self, appointment)
     42             'url': url
     43         })
---> 44         msg.send([user.email])
     45
     46         return user

/usr/local/lib/python3.6/site-packages/mail_factory/mails.py in send(self, emails, attachments, from_email, headers)
    151         """Create the message and send it to emails."""
    152         message = self.create_email_msg(emails, attachments=attachments,
--> 153                                         from_email=from_email, headers=headers)
    154         message.send()
    155

/usr/local/lib/python3.6/site-packages/mail_factory/mails.py in create_email_msg(self, emails, attachments, from_email, lang, message_class, headers)
    104
    105         from_email = from_email or settings.DEFAULT_FROM_EMAIL
--> 106         subject = self._render_part('subject.txt', lang=lang).strip()
    107         try:
    108             body = self._render_part('body.txt', lang=lang)

/usr/local/lib/python3.6/site-packages/mail_factory/mails.py in _render_part(self, part, lang)
     89
     90         """
---> 91         tpl = select_template(self.get_template_part(part, lang=lang))
     92         cur_lang = translation.get_language()
     93         try:

/usr/local/lib/python3.6/site-packages/mail_factory/mails.py in get_template_part(self, part, lang)
     69         templates = []
     70         # 1/ localized: mails/invitation_code/fr/
---> 71         localized = join('mails', self.template_name, lang or self.lang, part)
     72         templates.append(localized)
     73

/usr/local/lib/python3.6/posixpath.py in join(a, *p)
     92                 path += sep + b
     93     except (TypeError, AttributeError, BytesWarning):
---> 94         genericpath._check_arg_types('join', a, *p)
     95         raise
     96     return path

/usr/local/lib/python3.6/genericpath.py in _check_arg_types(funcname, *args)
    147         else:
    148             raise TypeError('%s() argument must be str or bytes, not %r' %
--> 149                             (funcname, s.__class__.__name__)) from None
    150     if hasstr and hasbytes:
    151         raise TypeError("Can't mix strings and bytes in path components") from None

TypeError: join() argument must be str or bytes, not 'NoneType'