While playing around with a freshly installed Qubes-OS 4.1.0
I discovered by accident that there are a few core commands
which make still use of the utility program zenity to display
error messages but the package zenity is not
installed in dom0 by default. For me the fix was easy:
sudo qubes-dom0-update zenity
I added the missing tool. Out of curiosity I had a look into
the sources and found at least three places which make use of
zenity::
So since kdialog is also not installed by default I believe this
could be considered as a bug in the default package selection.
Either zenity should be installed in dom0 by default or these three
places in qubes-core-admin-linux should be changed to some other
mechanism to display error messages.