only show related categories on category page

function filter_widget_categories_args( $args ) {
    if ( is_category() ) {
        $current_category = get_queried_object();
        if ( $current_category && ! is_wp_error( $current_category ) ) {
            $args['child_of'] = 0 !== $current_category->category_parent ? $current_category->category_parent : $current_category->term_id;

            if ( empty( get_categories( $args ) ) ) {
                unset( $args['child_of'] );
            }
        }
    }
    return $args;
}
add_filter( 'widget_categories_args', 'filter_widget_categories_args' );