Error while index OCA\FullTextSearch\Model\Index::setSource() must be of the type string, null given

[KossaKo]

After upgrading to Nextcloud 15 and Fulltextsearch 1.2 I encountered this error.
For me without any grasp of php it sounds remotely similar to #410 , however the fix did not help in that case.
Please let me know if I can provide more information.

Nextcloud 15.0.0
Fulltextsearch 1.2.0
Elasticsearch 6.5.3 with ingest-attachment
PHP 7.1.23
Mysql 10.1.26 (Mariadb)

An unhandled exception has been thrown:
TypeError: Argument 1 passed to OCA\FullTextSearch\Model\Index::setSource() must be of the type string, null given, called in /var/www/nextcloud/apps/fulltextsearch/lib/Db/IndexesRequestBuilder.php on line 114 and defined in /var/www/nextcloud/apps/fulltextsearch/lib/Model/Index.php:110
Stack trace:
#0 /var/www/nextcloud/apps/fulltextsearch/lib/Db/IndexesRequestBuilder.php(114): OCA\FullTextSearch\Model\Index->setSource(NULL)
#1 /var/www/nextcloud/apps/fulltextsearch/lib/Db/IndexesRequest.php(327): OCA\FullTextSearch\Db\IndexesRequestBuilder->parseIndexesSelectSql(Array)
#2 /var/www/nextcloud/apps/fulltextsearch/lib/Service/IndexService.php(274): OCA\FullTextSearch\Db\IndexesRequest->getIndexesFromProvider('files')
#3 /var/www/nextcloud/apps/fulltextsearch/lib/Service/IndexService.php(207): OCA\FullTextSearch\Service\IndexService->getProviderIndexFromProvider('files')
#4 /var/www/nextcloud/apps/fulltextsearch/lib/Service/IndexService.php(191): OCA\FullTextSearch\Service\IndexService->updateDocumentsWithCurrIndex(Object(OCA\Files_FullTextSearch\Provider\FilesProvider), Array, Object(OCA\FullTextSearch\Model\IndexOptions))
#5 /var/www/nextcloud/apps/fulltextsearch/lib/Command/Index.php(407): OCA\FullTextSearch\Service\IndexService->indexProviderContentFromUser(Object(OCA\FullTextSearch_ElasticSearch\Platform\ElasticSearchPlatform), Object(OCA\Files_FullTextSearch\Provider\FilesProvider), 'kossa', Object(OCA\FullTextSearch\Model\IndexOptions))
#6 /var/www/nextcloud/apps/fulltextsearch/lib/Command/Index.php(271): OCA\FullTextSearch\Command\Index->indexProvider(Object(OCA\Files_FullTextSearch\Provider\FilesProvider), Object(OCA\FullTextSearch\Model\IndexOptions))
#7 /var/www/nextcloud/3rdparty/symfony/console/Command/Command.php(255): OCA\FullTextSearch\Command\Index->execute(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#8 /var/www/nextcloud/core/Command/Base.php(166): Symfony\Component\Console\Command\Command->run(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#9 /var/www/nextcloud/3rdparty/symfony/console/Application.php(946): OC\Core\Command\Base->run(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#10 /var/www/nextcloud/3rdparty/symfony/console/Application.php(248): Symfony\Component\Console\Application->doRunCommand(Object(OCA\FullTextSearch\Command\Index), Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#11 /var/www/nextcloud/3rdparty/symfony/console/Application.php(148): Symfony\Component\Console\Application->doRun(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#12 /var/www/nextcloud/lib/private/Console/Application.php(213): Symfony\Component\Console\Application->run(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#13 /var/www/nextcloud/console.php(96): OC\Console\Application->run()
#14 /var/www/nextcloud/occ(11): require_once('/var/www/nextcl...')