Event registration breaks after updating to PHP8.1

In preparation for beginning the migration of a D7 site to D10+ I updated the PHP version from 7.4.x to 8.1.x. A Civi-integrated webform that lets users register for a couple of paid events subsequently stopped working.
The logs show a series of three PHP warnings followed by a fatal error:

Warning: Undefined array key "id" in wf_crm_webform_postprocess->processParticipants() (line 1091 of /home/mysite/sites/default/sites/all/modules/webform_civicrm/includes/wf_crm_webform_postprocess.inc).

Warning: Undefined array key "id" in wf_crm_webform_postprocess->processParticipants() (line 1097 of /home/mysite/sites/default/sites/all/modules/webform_civicrm/includes/wf_crm_webform_postprocess.inc).

Warning: Undefined array key "id" in wf_crm_webform_postprocess->processParticipants() (line 1105 of /home/mysite/sites/default/sites/all/modules/webform_civicrm/includes/wf_crm_webform_postprocess.inc).

TypeError: htmlentities(): Argument #1 ($string) must be of type string, array given in htmlentities() (line 473 of /home/inca/sites/default/sites/all/modules/civicrm/CRM/Utils/Type.php).

Drupal is at 7.101, Webform is 7.x-4.26, Webform_civicrm is 7.x-5.8 and CiviCRM is at 5.78.2