After exporting a property on an interface, that property will be
described, correctly, when a GetManagedObjects request is received from
another program in the system.

But if that GetManagedObjects method call is received while a
dbus-call-method request is waiting for a reply message, processing the
GetManagedObjects request signals an error.

I've attached three files:

- backtrace
- python test program
- elisp test script

The Python test program is a modified version of
https://gitlab.freedesktop.org/dbus/dbus-python/blob/master/examples/example-service.py.
It registers a method which is called by the elisp script.

Upon receipt of the elisp method call, the python program launches a
"dbus-send" command to issue a GetManagedObjects request back to the
source of the method call.  That generates the error described above.

To reproduce this problem:

- python2 /var/tmp/example-service.py
- emacs -Q --load ~/.config/emacs/dbus-mo-signal.el --eval "(main)"

That should invoke the debugger within ten seconds or so.

Thanks,
Hugh