array_walk error in MB Builder

Got errors:
Warning: array_walk() expects parameter 1 to be array, null given in .../wp-content/plugins/meta-box-builder/src/Upgrade.php on line 84

Warning: array_walk() expects parameter 1 to be array, null given in .../wp-content/plugins/meta-box-builder/src/Upgrade.php on line 85

They're displayed at the top of every post view. Even if it's really only a warning, it's not something I want to be showing.

Was doing lot's of work with MB today.
Added advanced attributes - pages were still working.
Added new fields - pages still working.
Applied the MB Builder update - pages still working.
Added two fields via MB Admin Columns. Did not verify pages, since didn't expect problems.

However, when I went back to work on adding custom fields to an old post, got the errors above when viewing it.

I tried undoing everything I had done, in reverse order. The errors persisted. BTW, purged all caches after each thing was undone.
Started deactivating the extension plugins, one at a time.
The error didn't go away until the only thing left was the basic free MetaBox.
Tried to activate MB Builder to verify it was the problem - and the errors came back.

Didn't see any errors on Google F12 diags.

What do I need to provide you to figure out what happened?
Right now, Builder and all extensions are disabled, and the work from earlier in the day is still removed.

thanks,
chris