When Html2Wiki is enabled with Mediawiki version 1.35.0-alpha (e7493e3) the following php fatal error is recorded:
[23-Feb-2020 00:56:15 UTC] PHP Fatal error: Uncaught UnexpectedValueException: callback 'SpecialHtml2Wiki::checkEnvironment' is not callable in /var/lib/mediawiki/includes/registration/ExtensionRegistry.php:536
Stack trace:
#0 /var/lib/mediawiki/includes/registration/ExtensionRegistry.php(236): ExtensionRegistry->exportExtractedData(Array)
#1 /var/lib/mediawiki/includes/Setup.php(141): ExtensionRegistry->loadFromQueue()
#2 /var/lib/mediawiki/includes/WebStart.php(89): require_once('/var/lib/mediaw...')
#3 /var/lib/mediawiki/index.php(44): require('/var/lib/mediaw...')
#4 {main}
thrown in /var/lib/mediawiki/includes/registration/ExtensionRegistry.php on line 536
~It looks like function SpecialHtml2Wiki::checkEnvironment is defined in extension.json but not implemented in SpecialHtml2Wiki.php