Same problem & solution, deactivated all plugins except ACF Pro, upgraded database, message was gone, re-activated plugins.
Fresh start in the day, noticed new WP version, updated, broke the Editor tab switching, right on the day when I have a ton of content editing to do.
Thank you so much Dennis, solved the Editor tab not switching issue.
For me the code was in line numbers 303 & 304. ACF Pro v5.2.9
Thanks Elliot, I can confirm, this attached version works.