SUMMARY
When a program asks you to pick a directory, you can't pick the directory that the file dialog is currently showing. Instead you must go to the parent directory, select the directory you just were in and then pick it.

STEPS TO REPRODUCE
0. Install DuckStation with Flatpak
1. Start DuckStation (Qt version) and open Settings
2. Select Game List Settings and click the small plus icon to add a Search Directory
3. Open any directory in the dialog and don't select any other file or directory
4. Click open

OBSERVED RESULT
Nothing happens

EXPECTED RESULT
The current directory is picked

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 
KDE Plasma Version: 5.22.3
KDE Frameworks Version: 5.84
Qt Version: 5.15.2