When last_version is deleted, do not delete text and previous versions in cascade, but really delete text only when there is no previous version, otherwise update text.last_version with previous version.
{% extends "site/layout/base.html" %}
{% load i18n %}
{% block content %}
<h1>404: {% blocktrans %}Page not found{% endblocktrans %}</h1>
{% blocktrans %}The requested page could not be found.{% endblocktrans %}
{% endblock %}