gnome-boxes from flatpak: file selection window does not show

I had that issue on gentoo running systemd with xfce4. Even though xdg-desktop-portal and xdg-desktop-portal-gtk portals were installed and running, gnome-boxes file selection window was not working. The solution was to set gtk portal by default:

create ~/.config/xdg-desktop-portal/portals.conf:

[preferred]
default=gtk
org.freedesktop.impl.portal.FileChooser=gtk