I've installed flatpak, have restarted my machine multiple times, have
uninstalled & reinstalled Flatpak.

Whenever I run flatpak update or flatpak run com.slack.Slack, I get the
following warning:

Note that the directories 

'/var/lib/flatpak/exports/share'
'/home/doug/.local/share/flatpak/exports/share'

are not in the search path set by the XDG_DATA_DIRS environment variable, so
applications installed by Flatpak may not appear on your desktop until the
session is restarted.

To log into Slack workspaces, Slack is supposed to open a URL in your browser,
you log in, and it is supposed to redirect you back to the application.

Another XDG problem pops up. When it gets to the point of redirecting you from
the website back to the program (after logging in) this error displays in
Chromium:

> Could not find any application or handler for slack://t1gh2byu9/magic-login/[CENSORED]

The behavior is similar in Firefox. But instead, it just doesn't show an error
at all and keeps on loading.