qdbusmenubar intermittently fails to export global menu

Sometimes (not sure what causes this to succeed or fail), a Qt application will respond to

method call time=1498266455.143512 sender=:1.34 -> destination=:1.138 serial=5010 path=/MenuBar/2; interface=org.freedesktop.DBus.Properties; member=GetAll

with

error time=1498266479.982908 sender=:1.138 -> destination=:1.34 error_name=org.freedesktop.DBus.Error.UnknownObject reply_serial=5010
   string "No such object path '/MenuBar/2'"

(see line 837 onwards of attached notworking.log). At other times, this succeeds.

This issue doesn't emerge with the appmenu-qt5 platformtheme, which uses a different implementation.

Bug reports about this:

• https://github.com/rilian-la-te/vala-panel-appmenu/issues/104 (pushes blame to qt5ct)
• https://sourceforge.net/p/qt5ct/tickets/42/ (pushes blame to Qt)