If I try to preview a previous revision of an entry, I get "an unknown error occurred" Craft 3.3.15

Previous version of the entry was saved 1 minute ago, so no changes in settings between the two entries.

Any ideas on how to debug this?

Update - here's the error, which looks very much like what @mtnorthrop is getting:

2019-11-20 10:41:44 [-][1][-][error][TypeError] TypeError: Argument 1 passed to craft\helpers\ElementHelper::isTempSlug() must be of the type string, null given, called in /Users/andrei/Desktop/dev/hl/mailbuilder/vendor/craftcms/cms/src/controllers/EntryRevisionsController.php on line 473 and defined in /Users/andrei/Desktop/dev/hl/mailbuilder/vendor/craftcms/cms/src/helpers/ElementHelper.php:47
This bug was due to an issue with the Stringify library and was fixed in Craft 3.3.16.1, as stated by Brad Bell in his comment on the original question.

Upgrading to Craft 3.3.16.1 fixes this issue.

(I'm posting this as an answer in case anybody else runs into the same issue.)