Impossible to invoke a method ("resize") on a null variable.
<div class="u-uppercase">{{ field.name }}</div>
<div>{{ additionalField.text.kirbytextinline|raw }}</div>
</div>
{% endfor %}
{% if page.cover.isEmpty() == false %}
<div class="u-cover">
{% set coverImage = page.cover.toFile().resize(0, 600) %}
<img alt="{{ coverImage.alt }}" src="{{ coverImage.url }}" width="{{ coverImage.width }}" height="600" />
</div>
{% endif %}
{% if page.downloads.isEmpty() == false %}
<div class="u-rowPadding">
<div class="u-uppercase">{{ t('downloads' )}}</div>