Illegal offset type DuplicateTitles.php:121

When running the Content check, in DuplicateTitles::calculateScore(), the method calls SafeMarkup::checkPlain() on the row title before using it as an index for the duplicate title registry.

The problem is that SafeMarkup::checkPlain() doesn't return a string it returns an HtmlEscapedText object. This is not a valid array key so an error is generated:

Illegal offset type DuplicateTitles.php:121

SafeMarkup::checkPlain() is deprecated and should be replaced with Html::escape(). Html::escape() returns a string rather than an HtmlEscapedText object so it works the same way as the D7 version of Site Audit.