Description of problem:

After running btrfs-assistant, flatpak apps would fail to launch with the error message: bwrap: Can't find source path /run/user/100/doc/by-app/<id of app>: Permission denied

Version-Release number of selected component: 2.1.1-3.fc42

How reproducible: Always

Steps to Reproduce:
1. Run btrfs-assistant, enter the user password to enter the main interface, and then close it.
2. Run any flatpak app.

Actual results:
Flatpak apps fail to launch. If ran from the command line, the following error message is printed: bwrap: Can't find source path /run/user/100/doc/by-app/<id of app>: Permission denied

Expected results: Flatpak app launches.

Additional info:

This has been reported upstream already on https://gitlab.com/btrfs-assistant/btrfs-assistant/-/issues/105 and https://github.com/flatpak/flatpak/issues/5860. It seems that this is only happening on Fedora. 

I observed 2 xdg-document-portal processes running, one as the user and another as root. Running `sudo killall xdg-document-portal` as suggested by some users in upstream bug reports resolved the problem for me.