Impossible to access an attribute ("url") on a null variable.
<div class="mt-6">
{% if page.images.count == 1 %}
<div class="mb-4 js-gallery">
<figure>
<a href="{{ page.cover.url }}" data-size="{{ page.cover.width ~ 'x' ~ page.cover.height }}">
{{ pattern('image', {
image: page.cover,
}) }}
</a>
</figure>
</div>