Flatpak nightly build is broken.
Here is the manifest.
How to reproduce:
flatpak remote-add --user --if-not-exists gnome-nightly https://sdk.gnome.org/gnome-nightly.flatpakrepo
flatpak remote-add --user --if-not-exists gnome-apps-nightly https://sdk.gnome.org/gnome-apps-nightly.flatpakrepo
curl -o org.gnome.Rhythmbox3.json "https://gitlab.gnome.org/GNOME/gnome-apps-nightly/raw/master/org.gnome.Rhythmbox3.json"
flatpak-builder --user --install --force-clean --repo=repo app org.gnome.Rhythmbox3.json
flatpak run org.gnome.Rhythmbox3//master
and here is the build log.
Edited by Jordan Petridis