Saving XFCE session with Gajim open hangs and causes Gajim to exit

Versions

OS: Ubuntu 20.04
Gajim version: Nightly 20200717-1 (from Gajim apt repo)
GTK version: 3.24.20-0ubuntu1
Python-nbxmpp version: Nightly 20200802-1 (from Gajim apt repo)

Steps to reproduce the problem

Start Gajim in an XFCE desktop session.
Open the XFCE "Session and startup" preference pane, go to "Current session" and hit "Save session".
Note that a progress dialog runs for ~1 minute (save is normally instant). When the progress dialog closes, Gajim exits.

Expected behavior

Gajim state gets saved with X session and Gajim is re-opened on login.

Actual behavior

Gajim state is not saved, instead the XFCE save dialog hangs for a minute and afterwards Gajim exits:

(org.gajim.Gajim:61038): Gtk-WARNING **: 14:41:03.362: Failed to unregister client: GDBus.Error:org.xfce.SessionManager.Error.General: Client with id of '/org/xfce/SessionClients/org_gajim_Gajim' was not found
08/09/20 14:41:03 (D) gajim.c.helpers            Save roster position: 97 0
08/09/20 14:41:03 (I) gajim.c.optparser          Successful saved config file
08/09/20 14:41:03 (I) gajim.c.optparser          Successful saved config file
08/09/20 14:41:03 (I) gajim.client               State: ClientState.DISCONNECTING
08/09/20 14:41:03 (I) gajim.client               Starting to disconnect <censored>
08/09/20 14:41:03 (W) gajim.client               Trying to send message while offline

(org.gajim.Gajim:61038): Gtk-WARNING **: 14:41:03.376: Can't set a target list on a widget until you've called gtk_drag_dest_set() to make the widget into a drag destination
08/09/20 14:41:03 (D) gajim.c.logger             Execution time for commit: 1 ms

No relevant logs seem to be printed when session save starts or while it is running. Session saving is working normally for other apps.

Edited Aug 09, 2020 by André