Typed property DOMDocument::$documentElement must not be accessed before initialization

I'm unable to reproduce, but one of Roundcube users created a ticket for this here.

This is with PHP 8.1.13:

PHP Fatal error:  Uncaught Error: Typed property DOMDocument::$documentElement must not be accessed before initialization in /var/www/roundcube/vendor/masterminds/html5/src/HTML5/Parser/DOMTreeBuilder.php:275
Stack trace:
#0 /var/www/roundcube/vendor/masterminds/html5/src/HTML5/Parser/Tokenizer.php(399): Masterminds\HTML5\Parser\DOMTreeBuilder->startTag()
#1 /var/www/roundcube/vendor/masterminds/html5/src/HTML5/Parser/Tokenizer.php(141): Masterminds\HTML5\Parser\Tokenizer->tagName()
#2 /var/www/roundcube/vendor/masterminds/html5/src/HTML5/Parser/Tokenizer.php(82): Masterminds\HTML5\Parser\Tokenizer->consumeData()
#3 /var/www/roundcube/vendor/masterminds/html5/src/HTML5.php(161): Masterminds\HTML5\Parser\Tokenizer->parse()
#4 /var/www/roundcube/vendor/masterminds/html5/src/HTML5.php(89): Masterminds\HTML5->parse()
#5 /var/www/roundcube/program/lib/Roundcube/rcube_washtml.php(698): Masterminds\HTML5->loadHTML()

Start in HTML5/Parser/DOMTreeBuilder.php around line 275, then trace the call path through Tokenizer.php and HTML5.php shown in the stack trace. Reproduce the reported PHP 8.1 failure if possible, and add a regression test demonstrating that parsing no longer accesses an uninitialized DOMDocument::$documentElement.