g_dbus_proxy_new_for_bus_sync: assertion 'g_variant_is_object_path (object_path)' failed

[Germano Massullo]

I am implementing some code (in attachment) that should retrieve
systemd-logind `IdleSinceHint` property exposed on DBus, but when it
is runned it fails on assertion `g_assert (proxy != NULL);` with error

    (process:9059): GLib-GIO-CRITICAL **: 17:29:07.245:
g_dbus_proxy_new_for_bus_sync: assertion 'g_variant_is_object_path
(object_path)' failed
    **
    ERROR:print_user_idle_time.c:44:main: assertion failed: (proxy != NULL)

What can be the problem?
Thank you for your time
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