I found a solution on this page-
https://www.ads-software.com/support/topic/read-this-first-wordpress-5-4-master-list/#post-12608712
Specifically this fix-
“Don’t Panic!
Before you go any further, make sure you’ve updated your plugins and themes to the latest versions, clear your browser’s cache and cookies and re-log in to your WordPress dashboard.
The New Editor can be disabled!
If you’re in a pinch and need the editor rolled back ASAP, please install the classic editor – www.ads-software.com/plugins/classic-editor – which will restore the old editor.”