FileDialog doesn't set fileUrl or fileURLs when run using XDG portals

FileDialog sets fileUrl and/or fileUrls when run normally and the user accepts the dialog. However, when executed in a Flatpak app where the file dialog comes from an XDG desktop portal, neither are set, so it is impossible to get the URL or URLs back from the dialog.

The attached test program illustrates the issue. When you compile and run it normally, show the dialog, type a filename, and accept, fileUrl and fileURLs get set. When you do the same in a Flatpak, they don't.

Local compilation:

cd filedialogfail && cmake . && make && ./filedialogfail

Flatpak compilation:

cd filedialogfail && flatpak-builder --force-clean --user --install build-dir org.nate.filedialogfail.yaml && flatpak run org.nate.filedialogfail