Crash defining resources

[clarkac1]

Using CiviCRM 5.26.2 with PHP 7.3 I get the following crash when defining a resource set "Fatal error: Access level to CRM_Admin_Form_ResourceConfigSet::$_id must be public (as in class CRM_Admin_Form) in /home/acivior1/public_html/sites/default/files/civicrm/ext/uk.co.compucorp.civicrm.booking/CRM/Admin/Form/ResourceConfigSet.php on line 40". This was fixed by commenting out line 41 in ResourceConfigSet (protected $_id = NULL;). Similar statements need to be removed in Resource.php and ResourceConfigOption.php. Civibooking version is 1.5.3