Search Error Appears on Blank Searches
{{ post.postImage.first().getUrl('thumbImage')}}


Whatever entry post is referencing for whatever search term you're using when you get the error, doesn't have any postImage assets selected.

So when you call post.postImage.first(), that returns null and you can't call getUrl on null.

You can either make postImage required and make sure that all existing entries have at least one asset selected, or guard against it in Twig via:

{% set image = post.postImage.first() %}

{% if image|length %}
    {{ image.getUrl('thumbImage') }}
{% endif %}