Needed to grant permission to access filesystem
After I installed this flatpak (either with --user or to system) it could not access files, either by opening them with Shotwell, dragging or dropping, or clicking Open in Shotwell.
To fix the issue I had to run:
flatpak --user override org.gnome.Shotwell --filesystem=host
I'm using Flatpak 1.3.4 on Arch Linux.