**List of steps to reproduce** (step by step, including full links if applicable):
* Got
to https://el.wikipedia.org/wiki/%CE%A4%CE%B9%CE%BC%CE%AE_%CF%83%CE%B7%CE%BC%CE%B1%CE%BD%CF%84%CE%B9%CE%BA%CF%8C%CF%84%CE%B7%CF%84%CE%B1%CF%82?uselang=en
**What happens?**:
* It returns: `[5dd28909-878f-419b-b4a2-69ab76702b20] 2021-08-13 18:16:28: Fatal exception of type "TypeError"`
* History and Page info return ok
```
TypeError: Argument 1 passed to MediaWiki\Extension\Math\HookHandlers\ParserHooksHandler::mathTagHook() must be of the type string, null given, called in /srv/mediawiki/php-1.37.0-wmf.18/includes/parser/Parser.php on line 3966
```
```name=exception.trace,lines=10
from /srv/mediawiki/php-1.37.0-wmf.18/extensions/Math/src/HookHandlers/ParserHooksHandler.php(71)
#0 /srv/mediawiki/php-1.37.0-wmf.18/includes/parser/Parser.php(3966): MediaWiki\Extension\Math\HookHandlers\ParserHooksHandler->mathTagHook(NULL, array, Parser, PPFrame_Hash)
#1 /srv/mediawiki/php-1.37.0-wmf.18/includes/parser/PPFrame_Hash.php(351): Parser->extensionSubstitution(array, PPFrame_Hash)
#2 /srv/mediawiki/php-1.37.0-wmf.18/includes/parser/Parser.php(2923): PPFrame_Hash->expand(PPNode_Hash_Tree, integer)
#3 /srv/mediawiki/php-1.37.0-wmf.18/includes/parser/Parser.php(1591): Parser->replaceVariables(string)
#4 /srv/mediawiki/php-1.37.0-wmf.18/includes/parser/Parser.php(652): Parser->internalParse(string)
#5 /srv/mediawiki/php-1.37.0-wmf.18/includes/content/WikitextContent.php(345): Parser->parse(string, Title, ParserOptions, boolean, boolean, integer)
#6 /srv/mediawiki/php-1.37.0-wmf.18/includes/content/AbstractContent.php(540): WikitextContent->fillParserOutput(Title, integer, ParserOptions, boolean, ParserOutput)
#7 /srv/mediawiki/php-1.37.0-wmf.18/includes/Revision/RenderedRevision.php(266): AbstractContent->getParserOutput(Title, integer, ParserOptions, boolean)
#8 /srv/mediawiki/php-1.37.0-wmf.18/includes/Revision/RenderedRevision.php(235): MediaWiki\Revision\RenderedRevision->getSlotParserOutputUncached(WikitextContent, boolean)
#9 /srv/mediawiki/php-1.37.0-wmf.18/includes/Revision/RevisionRenderer.php(217): MediaWiki\Revision\RenderedRevision->getSlotParserOutput(string, array)
#10 /srv/mediawiki/php-1.37.0-wmf.18/includes/Revision/RevisionRenderer.php(154): MediaWiki\Revision\RevisionRenderer->combineSlotOutput(MediaWiki\Revision\RenderedRevision, array)
#11 [internal function]: MediaWiki\Revision\RevisionRenderer->MediaWiki\Revision\{closure}(MediaWiki\Revision\RenderedRevision, array)
#12 /srv/mediawiki/php-1.37.0-wmf.18/includes/Revision/RenderedRevision.php(197): call_user_func(Closure, MediaWiki\Revision\RenderedRevision, array)
#13 /srv/mediawiki/php-1.37.0-wmf.18/includes/poolcounter/PoolWorkArticleView.php(137): MediaWiki\Revision\RenderedRevision->getRevisionParserOutput()
#14 /srv/mediawiki/php-1.37.0-wmf.18/includes/poolcounter/PoolCounterWork.php(162): PoolWorkArticleView->doWork()
#15 /srv/mediawiki/php-1.37.0-wmf.18/includes/page/ParserOutputAccess.php(281): PoolCounterWork->execute()
#16 /srv/mediawiki/php-1.37.0-wmf.18/includes/page/Article.php(746): MediaWiki\Page\ParserOutputAccess->getParserOutput(WikiPage, ParserOptions, MediaWiki\Revision\RevisionStoreCacheRecord, integer)
#17 /srv/mediawiki/php-1.37.0-wmf.18/includes/page/Article.php(558): Article->generateContentOutput(User, ParserOptions, integer, OutputPage, array)
#18 /srv/mediawiki/php-1.37.0-wmf.18/includes/actions/ViewAction.php(74): Article->view()
#19 /srv/mediawiki/php-1.37.0-wmf.18/includes/MediaWiki.php(538): ViewAction->show()
#20 /srv/mediawiki/php-1.37.0-wmf.18/includes/MediaWiki.php(320): MediaWiki->performAction(Article, Title)
#21 /srv/mediawiki/php-1.37.0-wmf.18/includes/MediaWiki.php(925): MediaWiki->performRequest()
#22 /srv/mediawiki/php-1.37.0-wmf.18/includes/MediaWiki.php(559): MediaWiki->main()
#23 /srv/mediawiki/php-1.37.0-wmf.18/index.php(53): MediaWiki->run()
#24 /srv/mediawiki/php-1.37.0-wmf.18/index.php(46): wfIndexMain()
#25 /srv/mediawiki/w/index.php(3): require(string)
#26 {main}
```