We currently assume that the OpenURI portal should be used
unconditionally when running inside a flatpak sandbox. While
the portal is what we usually want, there are exceptions:
Yelp is now included in the GNOME runtime to allow displaying
help without exporting the user documentation, and the sandboxed
app itself may register a scheme handler.
To account for those cases transparently, always try the normal
code path first and only fall back to calling the portal when
that fails.

https://bugzilla.gnome.org/show_bug.cgi?id=780471