Impossible to invoke a method ("crop") on a null variable.
{% if sliderItems|length > 0 %}
<div class="swiper-container js-swiper">
<div class="swiper-wrapper">
{% for sliderItem in sliderItems %}
<div class="swiper-slide" data-caption="{{ sliderItem.caption }}">
{% set image = sliderItem.image.toFile() %}
{{ image.crop(650, 430)|raw }}
</div>
{% endfor %}
</div>
<div class="swiper-footer">
<div class="swiper-pagination headline4"></div>
<div class="swiper-caption copySmall">{{ sliderItems.first().caption }}</div>