[Evolution] Error with Links (Flatpak build)

[Douglas Summers <ajgringo619 cox net>]

(Evolution 3.28 [flatpak], flatpak 0.11.7, Linux Mint 18.3)

Referencing this site (https://wiki.gnome.org/Apps/Evolution/Flatpak),
I built Evolution 3.28. Other than a few warnings, the build seemed to
go OK. After successfully creating an account, I tried to click on a
link within a message and got this error:

GDBus.Error: org.freedesktop.DBus.Error.ServiceUnknown: The name
org.freedesktop.portal.Desktop was not provided by any .service files.
In response to a post on the flatpak mailing list, I was told that I
needed to install the xdg-desktop-portal packages, but on both systems
they are present. This issue only happens with Evolution and not any
other flatpak apps.

Has anyone seen this? Mint only has Evolution 3.18; not exactly a new
release.