AttributeError: NoneType object has no attribute strip during link translation

[timobrembeck]

Describe the Bug

The new link translation feature introduced in #2403 seems to have a problem.

Steps to Reproduce

1. Go to /marburg-biedenkopf/pages/de-si/13080/edit/
2. Do something?
3. Save formn
4. See error

Expected Behavior

The form should be saved and the links should be automatically translated if the corresponding checkboxes are enabled

Actual Behavior

An internal server error occurs

Additional Information

Traceback

Oct 26 09:16:03 ERROR   django.request - 500 Internal Server Error: /marburg-biedenkopf/pages/de-si/13080/edit/
Traceback (most recent call last):
File "/opt/integreat-cms/.venv/lib/python3.9/site-packages/django/core/handlers/exception.py", line 47, in inner
  response = get_response(request)
File "/opt/integreat-cms/.venv/lib/python3.9/site-packages/django/core/handlers/base.py", line 181, in _get_response
  response = wrapped_callback(request, *callback_args, **callback_kwargs)
File "/opt/integreat-cms/.venv/lib/python3.9/site-packages/django/views/generic/base.py", line 70, in view
  return self.dispatch(request, *args, **kwargs)
File "/opt/integreat-cms/.venv/lib/python3.9/site-packages/django/utils/decorators.py", line 43, in _wrapper
  return bound_method(*args, **kwargs)
File "/opt/integreat-cms/.venv/lib/python3.9/site-packages/django/contrib/auth/decorators.py", line 21, in _wrapped_view
  return view_func(request, *args, **kwargs)
File "/opt/integreat-cms/.venv/lib/python3.9/site-packages/django/views/generic/base.py", line 98, in dispatch
  return handler(request, *args, **kwargs)
File "/usr/lib/python3.9/contextlib.py", line 79, in inner
  return func(*args, **kwds)
File "/opt/integreat-cms/.venv/lib/python3.9/site-packages/integreat_cms/cms/views/pages/page_form_view.py", line 305, in post
  if not page_form.is_valid() or not page_translation_form.is_valid():
File "/opt/integreat-cms/.venv/lib/python3.9/site-packages/django/forms/forms.py", line 175, in is_valid
  return self.is_bound and not self.errors
File "/opt/integreat-cms/.venv/lib/python3.9/site-packages/django/forms/forms.py", line 170, in errors
  self.full_clean()
File "/opt/integreat-cms/.venv/lib/python3.9/site-packages/django/forms/forms.py", line 372, in full_clean
  self._clean_fields()
File "/opt/integreat-cms/.venv/lib/python3.9/site-packages/django/forms/forms.py", line 393, in _clean_fields
  value = getattr(self, 'clean_%s' % name)()
File "/opt/integreat-cms/.venv/lib/python3.9/site-packages/integreat_cms/cms/forms/custom_content_model_form.py", line 132, in clean_content
  if translation := internal_link_utils.update_link_language(
File "/opt/integreat-cms/.venv/lib/python3.9/site-packages/integreat_cms/cms/utils/internal_link_utils.py", line 55, in update_link_language
  if source_translation.title.lower() == link_text.strip().lower():
AttributeError: 'NoneType' object has no attribute 'strip'

[timobrembeck]

The culprit was a link with HTML tags in the link text:

<a href="https://integreat.app/marburg-biedenkopf/de/sprache/sprachkurse/online-sprachkurse"><strong>Online-Angebote</strong></a>