gtk_notebook_get_tab_label: assertion 'list != NULL' failed

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There is already a thread on the Finnish board viewtopic.php?f=71&t=365153 relating to this error but mentioning xed.
However, it is not just limited to xed so I've started this one to avoid cross threading.

The errors are seen in .xession-errors and relate to multiple applications

MATE 20 - 20.2 Install with MATE 1.24 - no such errors
Update to 1.26 - errors
MATE 20.3 - MATE 1.26 lots of errors
Ubuntu MATE 21.10 MATE 1.26 - no such errors - though there are errors that look like they may be close relatives in a future life.

MATE 20.3 OOB with all updates - navigate through caja preferences tabs and you'll see these errors for caja.
As a bizarre side note if you install caja-admin the errors go away for caja

Error also shows if accessing properties / settings for Panel and Mate Power Manager

I do not think it is a particular issue and seems to crop up from time to time as a GTK bug going way back.