Oh the problem isn't if the backend supports the file picker or not, portals just weren't working for me properly. I had this problem when using Chromium for example, where the file dialog would never open. XDG portals should, if the backend doesn't have a file picker, fall back to the system's default one.
In my case I have moved `.Xauthority` file to a different place (AGES ago), and the portals backend would just refuse to work because it couldn't find the file (and it woulnd't fall back to the default one). I just had to tell the portals where the file was and it started working XD
Also I didn't notice the edit you made, or the fact that `popup_` methods didn't return anything for an exception. My bad on that xd.
I would also probably close or rewrite the github issue you left to something simple like "backup file dialog if system's file dialog breaks". It's not the issue with Godot or the game, but the OS's.
Sorry for getting this lengthy yet again, just trying to explain what's going on so there's no confusion xd