Impossible to invoke a method ("focusCrop") on a null variable.
{% block relatedContent %}
{% for residentialTalk in page.siblings(false).shuffle().limit(3) %}
<a class="related-link u-neutral-link" href="{{ residentialTalk.url }}" title="{{ residentialTalk.guest }}">
<article>
<h2 class="fontSmall">{{ residentialTalk.number }}</h2>
<div class="related-link__image">
{% set image = residentialTalk.content().image.toFile().focusCrop(580, 368) %}
<img alt="{{ image.alt }}" src="{{ image.url }}" alt width="580" height="368" />
</div>
<h3 class="fontMedium u-underline">{{ residentialTalk.guest }}</h3>
<h3 class="fontMedium u-underline">{{ residentialTalk.topic }}</h3>
</article>
</a>