v5.4.0 regression - .addClass() seems to created RSOD

Related to https://github.com/TiddlyWiki/TiddlyWiki5/pull/9251 It seems .addClass() is the culprit, but more testing needed. To reproduce - open [SampleWizard](https://tiddlywiki.com/#SampleWizard) - Add class field with: aaa bbb - Done - Open the PopUp -> OK -> Open nested popup -> RSOD