Twig\Error\RuntimeError

Line 29 of /var/www/wohnbau/site/templates/residential-talks.twig

Impossible to invoke a method ("focusCrop") on a null variable.

                    <div class="js-semester-content semesters__semester__content">
                        <div class="u-fourColumns">
                            {% for residentialTalk in semester.children() %}
                                <a class="related-link u-neutral-link" href="{{ residentialTalk.url }}" title="{{ residentialTalk.title }}">
                                    <article>
                                        <h2 class="fontSmall">{{ t('residentialTalk') }}</h2>
                                        <div class="related-link__image">{{ residentialTalk.cover.toFile().focusCrop(425, 600)|raw }}</div>
                                        <h3 class="fontMedium u-underline">{{ residentialTalk.guest }}</h3>
                                        <h3 class="fontMedium u-underline">{{ residentialTalk.topic }}</h3>
                                    </article>
                                </a>
                            {% endfor %}
                        </div>