SiteOrigin Blog Widget. Filter out non-top-level categories

snippet.php

<?php
add_filter( 'siteorigin_widgets_blog_filter_categories_output', function( $categories ) {
	if ( empty( $categories ) ) {
		return $categories;
	}

	return array_filter( $categories, function( $category ) {
		return $category->parent === 0;
	} );
}, 10, 1 );