Fulltextsearch indexing error /lib/private/Share20/Manager.php:1651

Nextcloud version 20.0.4
Full text search 21.0.3
Full text search - Elasticsearch Platform 21.0.2
Full text search - Files 21.0.4
elasticsearch 6.8.18
ingest-attachment plugin installed

sudo -u www-data php /var/www/nextcloud/occ fulltextsearch:test OK

Group folders enabled

While indexing starts after a while it throws this error:

An unhandled exception has been thrown:
Error: Call to a member function getOwner() on null in /var/www/nextcloud/lib/private/Share20/Manager.php:1651
Stack trace:
#0 /var/www/nextcloud/apps/files_fulltextsearch/lib/Service/LocalFilesService.php(163): OC\Share20\Manager->getAccessList()
#1 /var/www/nextcloud/apps/files_fulltextsearch/lib/Service/FilesService.php(795): OCA\Files_FullTextSearch\Service\LocalFilesService->getShareUsersFromFile()
#2 /var/www/nextcloud/apps/files_fulltextsearch/lib/Service/FilesService.php(725): OCA\Files_FullTextSearch\Service\FilesService->updateShareNames()
#3 /var/www/nextcloud/apps/files_fulltextsearch/lib/Service/FilesService.php(699): OCA\Files_FullTextSearch\Service\FilesService->updateDocumentAccess()
#4 /var/www/nextcloud/apps/files_fulltextsearch/lib/Service/FilesService.php(680): OCA\Files_FullTextSearch\Service\FilesService->updateFilesDocumentFromFile()
#5 /var/www/nextcloud/apps/files_fulltextsearch/lib/Service/FilesService.php(571): OCA\Files_FullTextSearch\Service\FilesService->updateFilesDocument()
#6 /var/www/nextcloud/apps/files_fulltextsearch/lib/Provider/FilesProvider.php(288): OCA\Files_FullTextSearch\Service\FilesService->generateDocument()
#7 /var/www/nextcloud/apps/fulltextsearch/lib/Service/IndexService.php(317): OCA\Files_FullTextSearch\Provider\FilesProvider->fillIndexDocument()
#8 /var/www/nextcloud/apps/fulltextsearch/lib/Service/IndexService.php(204): OCA\FullTextSearch\Service\IndexService->indexDocuments()
#9 /var/www/nextcloud/apps/fulltextsearch/lib/Command/Index.php(410): OCA\FullTextSearch\Service\IndexService->indexProviderContentFromUser()
#10 /var/www/nextcloud/apps/fulltextsearch/lib/Command/Index.php(273): OCA\FullTextSearch\Command\Index->indexProvider()
#11 /var/www/nextcloud/apps/bookmarks/vendor/symfony/console/Command/Command.php(255): OCA\FullTextSearch\Command\Index->execute()
#12 /var/www/nextcloud/core/Command/Base.php(169): Symfony\Component\Console\Command\Command->run()
#13 /var/www/nextcloud/apps/bookmarks/vendor/symfony/console/Application.php(1009): OC\Core\Command\Base->run()
#14 /var/www/nextcloud/apps/bookmarks/vendor/symfony/console/Application.php(273): Symfony\Component\Console\Application->doRunCommand()
#15 /var/www/nextcloud/apps/bookmarks/vendor/symfony/console/Application.php(149): Symfony\Component\Console\Application->doRun()
#16 /var/www/nextcloud/lib/private/Console/Application.php(215): Symfony\Component\Console\Application->run()
#17 /var/www/nextcloud/console.php(100): OC\Console\Application->run()
#18 /var/www/nextcloud/occ(11): require_once('/var/www/nextcl...')

sudo -u www-data php /var/www/nextcloud/occ fulltextsearch:check

Full text search 21.0.3
 
- Search Platform:
Elasticsearch 21.0.2 (Selected)
{
    "elastic_host": [
        "http://localhost:9200"
    ],
    "elastic_index": "ft_index",
    "fields_limit": "10000",
    "es_ver_below66": "0",
    "analyzer_tokenizer": "standard"
} 

- Content Providers:
Deck 1.4.3
[]
Files 21.0.4
{
    "files_local": "1",
    "files_external": "0",
    "files_group_folders": "1",
    "files_encrypted": "0",
    "files_federated": "0",
    "files_size": "20",
    "files_pdf": "1",
    "files_office": "1",
    "files_image": "0",
    "files_audio": "0"
}

Reproduce the failure with occ fulltextsearch:test and inspect lib/private/Share20/Manager.php:1651, starting from apps/files_fulltextsearch/lib/Service/LocalFilesService.php:163. Trace the share data through FilesService.php and determine why getOwner() is called on null. Done means indexing completes without this exception, including the group-folder configuration shown.