Fatal error on staff appointments list

Login as staff member and go to Appointments. This is what I got: Fatal error: Using $this when not in object context in /home/guide4gi/addond/fable/wp-content/plugins/woocommerce-appointments/includes/data-stores/class-wc-product-appointment-data-store-cpt.php on line 339

When accessing the Calendar in backend I get no calendar at all.