wireguard: Should not offer to backup before uninstall

The Wireguard app doesn't have a backup component or manifest. Normally, you can't take a backup for Wireguard. However, when uninstalling Wireguard, there is a prompt whether to take a backup. If it is selected, then there is an error:

   ERROR django.request       Internal Server Error: /plinth/uninstall/wireguard/
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/django/core/handlers/exception.py", line 47, in inner
    response = get_response(request)
  File "/usr/lib/python3/dist-packages/django/core/handlers/base.py", line 181, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/usr/lib/python3/dist-packages/django/views/generic/base.py", line 70, in view
    return self.dispatch(request, *args, **kwargs)
  File "/freedombox/plinth/views.py", line 410, in dispatch
    return super().dispatch(request, *args, **kwargs)
  File "/usr/lib/python3/dist-packages/django/views/generic/base.py", line 98, in dispatch
    return handler(request, *args, **kwargs)
  File "/usr/lib/python3/dist-packages/django/views/generic/edit.py", line 142, in post
    return self.form_valid(form)
  File "/freedombox/plinth/views.py", line 440, in form_valid
    repository.create_archive(name, [app_id])
  File "/freedombox/plinth/modules/backups/repository.py", line 183, in create_archive
    api.backup_apps(_backup_handler, path=archive_path, app_ids=app_ids,
  File "/freedombox/plinth/modules/backups/api.py", line 117, in backup_apps
    components = get_components_in_order(app_ids)
  File "/freedombox/plinth/modules/backups/api.py", line 216, in get_components_in_order
    components.append(_get_backup_restore_component(app_))
  File "/freedombox/plinth/modules/backups/api.py", line 194, in _get_backup_restore_component
    raise TypeError