You should check if you have testimonials ids
<?php
// get only first 1 results
$ids = get_field('testimonials_relationship', false, false);
// check if have ids
if($ids) {
$the_query = new WP_Query(array(
'post_type' => 'testimonials',
'posts_per_page' => 1,
'post__in' => $ids,
'post_status' => 'any',
'orderby' => 'rand',
));
?>
<?php if ( $the_query->have_posts() ) : ?>
<!-- pagination here -->
<!-- the loop -->
<?php while ( $the_query->have_posts() ) : $the_query->the_post(); ?>
<h2><?php the_title(); ?></h2>
<?php endwhile; ?>
<!-- end of the loop -->
<!-- pagination here -->
<?php wp_reset_postdata(); ?>
<?php else : ?>
<p><?php esc_html_e( 'Sorry, no posts matched your criteria.' ); ?></p>
<?php endif; ?>
}
