Error while creating index (Error: Call to a member function getOwner() on null in /var/www/nextcloud/lib/private/Share20/Manager.php:1651)

I'm getting the following error while creating the index:

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/3rdparty/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/3rdparty/symfony/console/Application.php(1009): OC\Core\Command\Base->run()
#14 /var/www/nextcloud/3rdparty/symfony/console/Application.php(273): Symfony\Component\Console\Application->doRunCommand()
#15 /var/www/nextcloud/3rdparty/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...')

This might be a problem with the app 'group folders' enabled.
This should have been fixed by https://github.com/nextcloud/server/pull/15832
But as a user already mentioned on the PR, the error seems to be back again in NC 21.

NC version: 21.0.3

Full text search 21.0.3
 
- Search Platform:
Elasticsearch 21.0.2 (Selected)
{
    "elastic_host": [
        "http://192.168.100.1:9200"
    ],
    "elastic_index": "my_index",
    "fields_limit": "10000",
    "es_ver_below66": "0",
    "analyzer_tokenizer": "standard"
} 

- Content Providers:
Files 21.0.4
{
    "files_local": "1",
    "files_external": "1",
    "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",
    "files_fulltextsearch_tesseract": {
        "version": "20.0.1",
        "enabled": "0",
        "psm": "4",
        "lang": "deu,eng",
        "pdf": "0",
        "pdf_limit": "0"
    }
}

.Testing your current setup:  
Creating mocked content provider. ok  
Testing mocked provider: get indexable documents. (2 items) ok  
Loading search platform. (Elasticsearch) ok  
Testing search platform. ok  
Locking process ok  
Removing test. ok  
Pausing 3 seconds 1 2 3 ok  
Initializing index mapping. ok  
Indexing generated documents. ok  
Pausing 3 seconds 1 2 3 ok  
Retreiving content from a big index (license). (size: 32386) ok  
Comparing document with source. ok  
Searching basic keywords:  
 - 'test' (result: 1, expected: ["simple"]) ok  
 - 'document is a simple test' (result: 2, expected: ["simple","license"]) ok  
 - '"document is a test"' (result: 0, expected: []) ok  
 - '"document is a simple test"' (result: 1, expected: ["simple"]) ok  
 - 'document is a simple -test' (result: 1, expected: ["license"]) ok  
 - 'document is a simple +test' (result: 1, expected: ["simple"]) ok  
 - '-document is a simple test' (result: 0, expected: []) ok  
 - 'document is a simple +test +testing' (result: 1, expected: ["simple"]) ok  
 - 'document is a simple +test -testing' (result: 0, expected: []) ok  
 - 'document is a +simple -test -testing' (result: 0, expected: []) ok  
 - '+document is a simple -test -testing' (result: 1, expected: ["license"]) ok  
 - 'document is a +simple -license +testing' (result: 1, expected: ["simple"]) ok  
Updating documents access. ok  
Pausing 3 seconds 1 2 3 ok  
Searching with group access rights:  
 - 'license' - [] -  (result: 0, expected: []) ok  
 - 'license' - ["group_1"] -  (result: 1, expected: ["license"]) ok  
 - 'license' - ["group_1","group_2"] -  (result: 1, expected: ["license"]) ok  
 - 'license' - ["group_3","group_2"] -  (result: 1, expected: ["license"]) ok  
 - 'license' - ["group_3"] -  (result: 0, expected: []) ok  
Searching with share rights:  
 - 'license' - notuser -  (result: 0, expected: []) ok  
 - 'license' - user2 -  (result: 1, expected: ["license"]) ok  
 - 'license' - user3 -  (result: 1, expected: ["license"]) ok  
Removing test. ok  
Unlocking process ok 

Start with lib/private/Share20/Manager.php:1651 and apps/files_fulltextsearch/lib/Service/LocalFilesService.php:163, following the getAccessList() call in the stack trace. Reproduce the failure while running the full-text index with group folders enabled, using the reported Nextcloud 21.0.3 setup. Done means indexing no longer crashes with a null getOwner() call.