Deleted comment entity leads to exceptions

Problem/Motivation

When a comment entity got deleted, DANSE may eventually throw exceptions like these:

TypeError: Drupal\danse\PluginBase::subscriptionKey(): Argument #2 must be of type string, null given, called in /var/www/html/web/modules/contrib/danse/modules/content/src/Payload.php on line 71 in /var/www/html/web/modules/contrib/danse/src/PluginBase.php on line 267 #0 /var/www/html/web/modules/contrib/danse/modules/content/src/Payload.php(71): Drupal\danse\PluginBase->subscriptionKey()
#1 /var/www/html/web/modules/contrib/danse/src/PluginBase.php(351): Drupal\danse_content\Payload->getSubscriptionReferences()
#2 /var/www/html/web/modules/contrib/danse/src/PluginBase.php(196): Drupal\danse\PluginBase->getSubscribers()
#3 /var/www/html/web/modules/contrib/danse/src/Service.php(311): Drupal\danse\PluginBase->createNotifications()
#4 /var/www/html/web/modules/contrib/danse/danse.module(29): Drupal\danse\Service->createNotifications()
#5 /var/www/html/web/core/lib/Drupal/Core/Cron.php(249): danse_cron()
#6 /var/www/html/web/core/lib/Drupal/Core/Extension/ModuleHandler.php(405): Drupal\Core\Cron->Drupal\Core\{closure}()
#7 /var/www/html/web/core/lib/Drupal/Core/Cron.php(257): Drupal\Core\Extension\ModuleHandler->invokeAllWith()
#8 /var/www/html/web/core/lib/Drupal/Core/Cron.php(136): Drupal\Core\Cron->invokeCronHandlers()
#9 /var/www/html/web/core/lib/Drupal/Core/ProxyClass/Cron.php(75): Drupal\Core\Cron->run()
#10 /var/www/html/vendor/drush/drush/src/Drupal/Commands/core/DrupalCommands.php(67): Drupal\Core\ProxyClass\Cron->run()
#11 [internal function]: Drush\Drupal\Commands\core\DrupalCommands->cron()
#12 /var/www/html/vendor/consolidation/annotated-command/src/CommandProcessor.php(257): call_user_func_array()
#13 /var/www/html/vendor/consolidation/annotated-command/src/CommandProcessor.php(212): Consolidation\AnnotatedCommand\CommandProcessor->runCommandCallback()
#14 /var/www/html/vendor/consolidation/annotated-command/src/CommandProcessor.php(176): Consolidation\AnnotatedCommand\CommandProcessor->validateRunAndAlter()
#15 /var/www/html/vendor/consolidation/annotated-command/src/AnnotatedCommand.php(390): Consolidation\AnnotatedCommand\CommandProcessor->process()
#16 /var/www/html/vendor/symfony/console/Command/Command.php(255): Consolidation\AnnotatedCommand\AnnotatedCommand->execute()
#17 /var/www/html/vendor/symfony/console/Application.php(1039): Symfony\Component\Console\Command\Command->run()
#18 /var/www/html/vendor/symfony/console/Application.php(275): Symfony\Component\Console\Application->doRunCommand()
#19 /var/www/html/vendor/symfony/console/Application.php(149): Symfony\Component\Console\Application->doRun()
#20 /var/www/html/vendor/drush/drush/src/Runtime/Runtime.php(124): Symfony\Component\Console\Application->run()
#21 /var/www/html/vendor/drush/drush/src/Runtime/Runtime.php(51): Drush\Runtime\Runtime->doRun()
#22 /var/www/html/vendor/drush/drush/drush.php(77): Drush\Runtime\Runtime->run()
#23 /var/www/html/vendor/drush/drush/includes/preflight.inc(18): require('...')
#24 phar:///usr/local/bin/drush/bin/drush.php(143): drush_main()
#25 /usr/local/bin/drush(14): require('...')
#26 {main}

Proposed resolution

Avoid this type of exception by asserting the referenced entity to be available.