and thus the selected doc can't be used as the source of external
data for this sheet. In other words, help the user figure out
why is OK button disabled

Change-Id: Ie1b999c9c3640397e9f6f6ee321d3a5a5d63fa3b
Reviewed-on: https://gerrit.libreoffice.org/72075
Tested-by: Jenkins
Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>