Admin interface fails when trying to create a new session

[jandd]

Describe the bug
When https://www.devday.de/admin/talk/talk/add/ -> 500 Internal Server Error

To Reproduce
Steps to reproduce the behavior:

1. As admin go to 'https://www.devday.de/admin/talk/talk/'
2. Click on https://www.devday.de/admin/talk/talk/add/
3. See error

Expected behavior
The form to create a new session is shown

Additional context

AttributeError at /admin/talk/talk/add/
'NoneType' object has no attribute 'event'
Request Method:	GET
Request URL:	https://www.devday.de/admin/talk/talk/add/
Django Version:	2.2.11
Exception Type:	AttributeError
Exception Value:	'NoneType' object has no attribute 'event'
Exception Location:	/app/talk/admin.py in formfield_for_foreignkey, line 94
Python Executable:	/usr/sbin/uwsgi
Python Version:	3.8.2
Python Path:	['/app/',
 '.',
 '',
 '/usr/lib/python38.zip',
 '/usr/lib/python3.8',
 '/usr/lib/python3.8/lib-dynload',
 '/usr/lib/python3.8/site-packages',
 '/python-code/src/djangocms-column']

[jandd]

The issue above is fixed in #289 but another issue has surfaced.

When saving the session we get another 404 due to a missing event_id in

  File "/app/talk/models.py", line 118, in save
    super(Talk, self).save(force_insert, force_update, using, update_fields)

which leads to:

django.db.utils.IntegrityError: null value in column "event_id" of relation "talk_talk" violates not-null constraint