DoesNotExist: Ranking matching query does not exist.

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https://sentry.jawne.info.pl/sentry/raportysiecobywatelskapl/issues/1148/

DoesNotExist: Ranking matching query does not exist.
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  File "raporty_siecobywatelska_pl/ranking/middleware.py", line 12, in process_view
    request.ranking = Ranking.objects.get_current(request)
  File "raporty_siecobywatelska_pl/ranking/models.py", line 20, in get_current
    return self.get(slug=ranking_slug)

DoesNotExist: Ranking matching query does not exist.