Return a title in the Bolt 404 page with SEO extension

master_with_404_title.twig

{% if app.request.get('_route') %}{# no route, no SEO tags #}
    <title>{{ seo.title() }}</title>
    {{ seo.metatags() }}
{% else %}
    <title>Niet Gevonden | {{ config.get('general/sitename') }}</title>
{% endif %}