Parser::$ot must not be accessed before initialization

[prudloff-insite]

We upgrade to MediaWiki 1.43 with semantic-forms-select 4.0.x-dev and now requests to /api.php?action=sformsselect crash with this error:

[eb45b002750ab17dc1011398] Exception caught: Typed property MediaWiki\\Parser\\Parser::$ot must not be accessed before initialization

Error at /srv/www/archi-mediawiki-test/core/includes/parser/Parser.php(3052)
from /srv/www/archi-mediawiki-test/core/includes/parser/Parser.php(3052)
#0 /srv/www/archi-mediawiki-test/core/includes/parser/PPFrame_Hash.php(280): MediaWiki\\Parser\\Parser->braceSubstitution()
#1 /srv/www/archi-mediawiki-test/core/includes/parser/Parser.php(2951): MediaWiki\\Parser\\PPFrame_Hash->expand()
#2 /srv/www/archi-mediawiki-test/extensions/SemanticFormsSelect/src/ApiSemanticFormsSelectRequestProcessor.php(145): MediaWiki\\Parser\\Parser->replaceVariables()
#3 /srv/www/archi-mediawiki-test/extensions/SemanticFormsSelect/src/ApiSemanticFormsSelectRequestProcessor.php(77): SFS\\ApiSemanticFormsSelectRequestProcessor->doProcessFunctionFor()
#4 /srv/www/archi-mediawiki-test/extensions/SemanticFormsSelect/src/ApiSemanticFormsSelect.php(33): SFS\\ApiSemanticFormsSelectRequestProcessor->getJsonDecodedResultValuesForRequestParameters()
#5 /srv/www/archi-mediawiki-test/core/includes/api/ApiMain.php(1974): SFS\\ApiSemanticFormsSelect->execute()
#6 /srv/www/archi-mediawiki-test/core/includes/api/ApiMain.php(942): MediaWiki\\Api\\ApiMain->executeAction()
#7 /srv/www/archi-mediawiki-test/core/includes/api/ApiMain.php(913): MediaWiki\\Api\\ApiMain->executeActionWithErrorHandling()
#8 /srv/www/archi-mediawiki-test/core/includes/api/ApiEntryPoint.php(153): MediaWiki\\Api\\ApiMain->execute()
#9 /srv/www/archi-mediawiki-test/core/includes/MediaWikiEntryPoint.php(200): MediaWiki\\Api\\ApiEntryPoint->execute()
#10 /srv/www/archi-mediawiki-test/core/api.php(44): MediaWiki\\MediaWikiEntryPoint->run()
#11 {main}