AttributeError when cloning region without imprint

[timobrembeck]

Describe the Bug

When a region is cloned that has no imprint, an internal server error occurs.
The region is created, but the duplication process is not completed.

Steps to Reproduce

1. Clone region without imprint (e.g. Nürnberg in the test data)
2. See error

Expected Behavior

The region should be cloned and all contents should be copied

Actual Behavior

AttributeError: 'NoneType' object has no attribute 'region'

Additional Information

Traceback

Internal Server Error: /regions/new/
Traceback (most recent call last):
File "/home/timo/job/integreat/integreat-cms/.venv/lib/python3.10/site-packages/django/core/handlers/exception.py", line 47, in inner
  response = get_response(request)
File "/home/timo/job/integreat/integreat-cms/.venv/lib/python3.10/site-packages/django/core/handlers/base.py", line 181, in _get_response
  response = wrapped_callback(request, *callback_args, **callback_kwargs)
File "/home/timo/job/integreat/integreat-cms/.venv/lib/python3.10/site-packages/django/views/generic/base.py", line 70, in view
  return self.dispatch(request, *args, **kwargs)
File "/home/timo/job/integreat/integreat-cms/.venv/lib/python3.10/site-packages/django/contrib/auth/mixins.py", line 104, in dispatch
  return super().dispatch(request, *args, **kwargs)
File "/home/timo/job/integreat/integreat-cms/.venv/lib/python3.10/site-packages/django/views/generic/base.py", line 98, in dispatch
  return handler(request, *args, **kwargs)
File "/home/timo/job/integreat/integreat-cms/.venv/lib/python3.10/site-packages/django/views/generic/edit.py", line 172, in post
  return super().post(request, *args, **kwargs)
File "/home/timo/job/integreat/integreat-cms/.venv/lib/python3.10/site-packages/django/views/generic/edit.py", line 142, in post
  return self.form_valid(form)
File "/home/timo/job/integreat/integreat-cms/integreat_cms/cms/views/form_views.py", line 127, in form_valid
  return super().form_valid(form)
File "/home/timo/job/integreat/integreat-cms/.venv/lib/python3.10/site-packages/django/views/generic/edit.py", line 125, in form_valid
  self.object = form.save()
File "/home/timo/job/integreat/integreat-cms/integreat_cms/cms/forms/regions/region_form.py", line 211, in save
  duplicate_imprint(source_region, region)
File "/home/timo/job/integreat/integreat-cms/integreat_cms/cms/forms/regions/region_form.py", line 733, in duplicate_imprint
  target_imprint.region = target_region
AttributeError: 'NoneType' object has no attribute 'region'