New clean install with
Semantic MediaWiki 3.2.0 installed from composer "mediawiki/semantic-media-wiki": "~3.1"
Page Forms 4.9.5 installed from archive https://github.com/wikimedia/mediawiki-extensions-PageForms/archive/4.9.5.zip
Disabling semantic wiki and the Create a class page works just fine.
[X2iNplZguz0HkStX-7tYfgAAAME] /index.php?title=Special:CreateClass Error from line 219 of /home/mediawiki/mediawiki-1.34.2/extensions/PageForms/specials/PF_CreateClass.php: Call to a member function getDatatypeLabels() on null Backtrace: #0 /home/mediawiki/mediawiki-1.34.2/includes/specialpage/SpecialPage.php(575): PFCreateClass->execute() #1 /home/mediawiki/mediawiki-1.34.2/includes/specialpage/SpecialPageFactory.php(611): SpecialPage->run() #2 /home/mediawiki/mediawiki-1.34.2/includes/MediaWiki.php(296): MediaWiki\Special\SpecialPageFactory->executePath() #3 /home/mediawiki/mediawiki-1.34.2/includes/MediaWiki.php(900): MediaWiki->performRequest() #4 /home/mediawiki/mediawiki-1.34.2/includes/MediaWiki.php(527): MediaWiki->main() #5 /home/mediawiki/mediawiki-1.34.2/index.php(44): MediaWiki->run() #6 {main}