Edit to UnixCluster with changed default group throws 500

Editing an existing UnixCluster and changing the Default Group from "Create a New Group For Each Account" to another choice throws a 500 with this stack trace:

2021-08-03 14:59:04 Error: [Error] Call to a member function findCoForRecord() on null
Request URL: /registry/unix_cluster/unix_clusters/edit/1
Stack Trace:
#0 /srv/comanage-registry/lib/Cake/Event/CakeEventManager.php(242): AppModel->beforeValidate(Array)
#1 /srv/comanage-registry/lib/Cake/Model/ModelValidator.php(454): CakeEventManager->dispatch(Object(CakeEvent))
#2 /srv/comanage-registry/lib/Cake/Model/ModelValidator.php(242): ModelValidator->_triggerBeforeValidate(Array)
#3 /srv/comanage-registry/lib/Cake/Model/ModelValidator.php(99): ModelValidator->errors(Array)
#4 /srv/comanage-registry/lib/Cake/Model/Model.php(3502): ModelValidator->validates(Array)
#5 /srv/comanage-registry/lib/Cake/Model/ModelValidator.php(134): Model->validates(Array)
#6 /srv/comanage-registry/lib/Cake/Model/Model.php(2659): ModelValidator->validateAssociated(Array, Array)
#7 /srv/comanage-registry/lib/Cake/Model/Model.php(2463): Model->validateAssociated(Array, Array)
#8 /srv/comanage-registry/lib/Cake/Model/Model.php(2287): Model->saveAssociated(Array, Array)
#9 /srv/comanage-registry/app/Controller/StandardController.php(650): Model->saveAll(Array)
#10 [internal function]: StandardController->edit('1')
#11 /srv/comanage-registry/lib/Cake/Controller/Controller.php(499): ReflectionMethod->invokeArgs(Object(UnixClustersController), Array)
#12 /srv/comanage-registry/lib/Cake/Routing/Dispatcher.php(193): Controller->invokeAction(Object(CakeRequest))
#13 /srv/comanage-registry/lib/Cake/Routing/Dispatcher.php(167): Dispatcher->_invoke(Object(UnixClustersController), Object(CakeRequest))
#14 /srv/comanage-registry/app/webroot/index.php(96): Dispatcher->dispatch(Object(CakeRequest), Object(CakeResponse))
#15 {main}