Massimo Valentini authored Oct 20, 2012 and Michael Natterer committed Oct 22, 2012

It seems that when the undo action reverts a text -> mark-up change
(or v.v.)  the notifications are inverted, first is emitted the new
value notification and successively the property becoming NULL. The
result is that gimp_text_buffer_set_{text,mark-up} is called last with
NULL, unsetting both text_buffer text and mark-up.

The right way to do it is to always prefer "markup" over "text" if
markup is present, no matter if the notification was for "markup" or
"text".