There is no error handling in PageRestriction::newFromTitle and perhaps there should be.
$ ag "PageRestriction::newFromTitle" includes/specials/SpecialBlock.php 829: $pageRestrictions[] = PageRestriction::newFromTitle( $title ); includes/api/ApiBlock.php 135: $pageRestrictions[] = PageRestriction::newFromTitle( $title );
Some other things to think about (from https://gerrit.wikimedia.org/r/c/mediawiki/core/+/676290/4/includes/block/BlockUser.php#441):
Perhaps we should keep this scoped to nonexistent titles and file the issue with invalid titles separately (thanks for raising it, Umherirrender). We might well want to make changes to PageRestriction::newFromTitle... We might even want to move creating the Restriction objects to BlockUser rather than repeating this in the API and the Special Page...