Steps to reproduce:
Visit https://en.wikipedia.org/wiki/Wikipedia:Scripts%2B%2B/Subscribe?action=edit (or probably any other mass message list)
Make the json invalid (eg remove the first {)
Click "Show changes"
Actual result:
[8b788f40-eff9-4c15-a1f1-9d7ab39070fd] 2020-12-03 23:47:58: Fatal exception of type "Wikimedia\Assert\ParameterTypeException"
trace
#0 /srv/mediawiki/php-1.36.0-wmf.18/includes/diff/TextSlotDiffRenderer.php(138): Wikimedia\Assert\Assert::parameterType(array, NULL, string)
#1 /srv/mediawiki/php-1.36.0-wmf.18/includes/diff/DifferenceEngine.php(1466): TextSlotDiffRenderer->getTextDiff(string, NULL)
#2 /srv/mediawiki/php-1.36.0-wmf.18/extensions/MassMessage/includes/Content/MassMessageListDiffEngine.php(36): DifferenceEngine->generateTextDiffBody(string, NULL)
#3 /srv/mediawiki/php-1.36.0-wmf.18/includes/diff/DifferenceEngineSlotDiffRenderer.php(54): MediaWiki\MassMessage\Content\MassMessageListDiffEngine->generateContentDiffBody(MediaWiki\MassMessage\Content\MassMessageListContent, MediaWiki\MassMessage\Content\MassMessageListContent)
#4 /srv/mediawiki/php-1.36.0-wmf.18/includes/diff/DifferenceEngine.php(1251): DifferenceEngineSlotDiffRenderer->getDiff(MediaWiki\MassMessage\Content\MassMessageListContent, MediaWiki\MassMessage\Content\MassMessageListContent)
#5 /srv/mediawiki/php-1.36.0-wmf.18/includes/diff/DifferenceEngine.php(1156): DifferenceEngine->getDiffBody()
#6 /srv/mediawiki/php-1.36.0-wmf.18/includes/EditPage.php(3765): DifferenceEngine->getDiff(string, string)
#7 /srv/mediawiki/php-1.36.0-wmf.18/includes/EditPage.php(3675): EditPage->showDiff()
#8 /srv/mediawiki/php-1.36.0-wmf.18/includes/EditPage.php(3219): EditPage->displayPreviewArea(string, boolean)
#9 /srv/mediawiki/php-1.36.0-wmf.18/includes/EditPage.php(717): EditPage->showEditForm()
#10 /srv/mediawiki/php-1.36.0-wmf.18/includes/actions/EditAction.php(71): EditPage->edit()
#11 /srv/mediawiki/php-1.36.0-wmf.18/includes/actions/SubmitAction.php(38): EditAction->show()
#12 /srv/mediawiki/php-1.36.0-wmf.18/includes/MediaWiki.php(532): SubmitAction->show()
#13 /srv/mediawiki/php-1.36.0-wmf.18/includes/MediaWiki.php(316): MediaWiki->performAction(Article, Title)
#14 /srv/mediawiki/php-1.36.0-wmf.18/includes/MediaWiki.php(945): MediaWiki->performRequest()
#15 /srv/mediawiki/php-1.36.0-wmf.18/includes/MediaWiki.php(548): MediaWiki->main()
#16 /srv/mediawiki/php-1.36.0-wmf.18/index.php(53): MediaWiki->run()
#17 /srv/mediawiki/php-1.36.0-wmf.18/index.php(46): wfIndexMain()
#18 /srv/mediawiki/w/index.php(3): require(string)
#19 {main}