Can't change preferred apps on Xubuntu 20.04 after upgrade from 18.04

[MR ZenWiz]

I recently upgraded my partner's laptop from Xubuntu 18.04, which was
running nicely, to 20.04.  For the most part it works fine.

However, today I went in to change the preferred web browser from
Mozilla Firefox to Google Chrome, and the change does not stick, or
even take hold.  It never changes from Firefox.  I tried to change the
mail reader to Chrome also, and hit the same wall.

On my desktop, on which I installed a fresh 20.04, I have no trouble
changing the preferred apps - I have chrome for both browser and
reader.

I notice in the preferred apps listing it shows Debian Sensible
browser and chrome, but both have the "not" slashed circle over them
inside a black rectangle.  I would guess that this is part of the
problem, but I also tried selecting "Other" and entered the FQPN of
the Chrome executable (/opt/google/chrome/chrome - I also tried
google-chrome as the leaf) and that also does not work.

WTH is going on here?  What do I have to do to change this setting?
It "should" be straightforward and just offer me the options among
what's installed, but that doesn't seem to work.

Thanks.

Mark