Source

File: wp-includes/widgets/class-wp-widget-pages.php

public function update( $new_instance, $old_instance ) {
		$instance          = $old_instance;
		$instance['title'] = sanitize_text_field( $new_instance['title'] );
		if ( in_array( $new_instance['sortby'], array( 'post_title', 'menu_order', 'ID' ), true ) ) {
			$instance['sortby'] = $new_instance['sortby'];
		} else {
			$instance['sortby'] = 'menu_order';
		}

		$instance['exclude'] = sanitize_text_field( $new_instance['exclude'] );

		return $instance;
	}