I have tried multiple flatpak apps that use a filepicker to pick a file like Popsicle and CherryTree but both could not get the file that was selected using the filepicker.
I’m on Fedora 35 KDE spin, and had the same issue back when I used Kubuntu.
Some possibly useful terminal output:
F: Can't get document portal: GDBus.Error:org.freedesktop.DBus.Error.NameHasNoOwner: Could not activate remote peer.
** (process:2): WARNING **: 13:10:13.579: locale::facet::_S_create_c_locale name not valid
(process:2): Gtk-WARNING **: 13:10:13.583: Locale not supported by C library.
Using the fallback 'C' locale.
[2022-04-04 13:10:13.591] [gtk] [critical] Class::register_derived_type(): base_query.type_name is NULL.
[2022-04-04 13:10:13.626] [ ] [debug] /home/<my username>/.var/app/com.giuspen.cherrytree/config/cherrytree/config.cfg parsed