When I build the master version via Flatpak in Builder, no music is found. Here is the output:

(flatpak build:4578): flatpak-DEBUG: Opening user flatpak installation at path /home/aplazas/.local/share/flatpak
(flatpak build:4578): flatpak-DEBUG: No installations directory in /etc/flatpak/installations.d. Skipping
(flatpak build:4578): flatpak-DEBUG: Disallowing network access
(flatpak build:4578): flatpak-DEBUG: Allowing host-fs access
(flatpak build:4578): flatpak-DEBUG: Allowing x11 access
(flatpak build:4578): flatpak-DEBUG: Allowing wayland access
(flatpak build:4578): flatpak-DEBUG: Allowing pulseaudio access
(flatpak build:4578): flatpak2-DEBUG: bwrap args '--ro-bind /etc /etc --ro-bind /.snapshots /.snapshots --ro-bind /boot /boot --ro-bind /opt /opt --ro-bind /srv /srv --bind /tmp /tmp --ro-bind /usr /usr --bind /var /var --ro-bind /data /data --ro-bind /home /home --ro-bind /dev /dev --ro-bind /proc /proc --ro-bind /sys /sys --bind /run /run --ro-bind /bin /bin --ro-bind /lib /lib --ro-bind /lib64 /lib64 --ro-bind /mnt /mnt --ro-bind /root /root --ro-bind /sbin /sbin --ro-bind /selinux /selinux --ro-bind /share /share --bind /run/user/1000/.dbus-proxy/ /run/user/1000/.dbus-proxy/ --file 17 /.flatpak-info'
(flatpak build:4578): flatpak2-DEBUG: Running '/usr/lib/flatpak-bwrap --args 13 /usr/lib/flatpak-dbus-proxy --fd=16 unix:path=/run/user/1000/bus /run/user/1000/.dbus-proxy/session-bus-proxy-ZOXRAZ --filter --own=org.gnome.Music '--own=org.gnome.Music.*' --talk=ca.desrt.dconf --talk=com.intel.dleyna-server --talk=org.freedesktop.Tracker1 '--talk=org.freedesktop.portal.*''

** (gnome-music:2): WARNING **: 11:42:32.712: Couldn't connect to accessibility bus: Failed to connect to socket /tmp/dbus-nn3BdZ6tBR: Connection refused
/usr/lib/python3.5/site-packages/gi/module.py:176: Warning: g_array_append_vals: assertion 'array' failed
  g_type = info.get_g_type()
/usr/lib/python3.5/site-packages/gi/module.py:176: Warning: g_hash_table_lookup: assertion 'hash_table != NULL' failed
  g_type = info.get_g_type()
/usr/lib/python3.5/site-packages/gi/module.py:176: Warning: g_hash_table_insert_internal: assertion 'hash_table != NULL' failed
  g_type = info.get_g_type()

(gnome-music:2): Tracker-WARNING **: 11:42:32.793: Locale 'LANG' is not set, defaulting to C locale
Tracker-Message: 11:42:32.794: Could not get mtime for 'file:///home/aplazas/.var/app/org.gnome.Music/cache/tracker/meta.db': Error when getting information for file “/home/aplazas/.var/app/org.gnome.Music/cache/tracker/meta.db”: No such file or directory

(gnome-music:2): Tracker-WARNING **: 11:42:32.794: Falling back to bus backend, the direct backend failed to initialize: Could not open sqlite3 database:'/home/aplazas/.var/app/org.gnome.Music/cache/tracker/meta.db': unable to open database file
11:42:32 WARNING	XDG Music dir is not set
11:42:33 WARNING	XDG Music dir is not set
/app/lib/python3.5/site-packages/gnomemusic/window.py:158: PyGTKDeprecationWarning: Using positional arguments with the GObject constructor has been deprecated. Please specify keyword(s) for "label" or use a class specific constructor. See: https://wiki.gnome.org/PyGObject/InitializerDeprecations
  self._playlist_notification.label = Gtk.Label('')

(gnome-music:2): Tracker-CRITICAL **: 11:42:33.367: tracker_bus_fd_cursor_real_get_string: assertion 'column < n_columns && data != null' failed

(gnome-music:2): Tracker-CRITICAL **: 11:42:33.368: tracker_bus_fd_cursor_real_get_string: assertion 'column < n_columns && data != null' failed

(gnome-music:2): Tracker-CRITICAL **: 11:42:33.371: tracker_bus_fd_cursor_real_get_string: assertion 'column < n_columns && data != null' failed

(gnome-music:2): Tracker-CRITICAL **: 11:42:33.371: tracker_bus_fd_cursor_real_get_string: assertion 'column < n_columns && data != null' failed

(gnome-music:2): Tracker-CRITICAL **: 11:42:33.372: tracker_bus_fd_cursor_real_get_string: assertion 'column < n_columns && data != null' failed

(gnome-music:2): Grilo-WARNING **: 11:42:33.373: [tracker-source-request] grl-tracker-source-api.c:504: Could not execute sparql query id=1: GDBus.Error:org.freedesktop.Tracker1.SparqlError.Internal: file is not a database

(gnome-music:2): Grilo-WARNING **: 11:42:34.374: [tracker-notif] grl-tracker-source-notif.c:216: Error: GDBus.Error:org.freedesktop.Tracker1.SparqlError.Internal: file is not a database