Fatal error

Hello
We use the latest version and checkout via rest api, and get the error on checkout:
PHP Fatal error: Uncaught Error: Call to a member function __unset() on null in …/plugins/order-tip-woo/frontend/controllers/main.class.php:218

main.class.php code in question:

if( $this->settings[‘wc_order_tip_remove_new_order’] && ! is_admin() ) {
$wc_session = WC()->session;
$wc_session->__unset( ‘tip’ );
}

According to this ticket https://wordpress.org/support/topic/fatal-error-3775/ it should be fixed…

Maybe something simple like:
if( isset( WC()->session ) ){
…
}
can do the trick?

Thank you.