could not register service org.freedesktop.Notifications on D-Bus

Hi, when I run example https://doc.qt.io/QtApplicationManager/qtapplicationmanager-applicationmanager-hello-world-example.html on a Ubuntu desktop platform,
the example works look fine. but it output

[CRIT | am.system] ERROR: could not register service org.freedesktop.Notifications on D-Bus (session): 
[WARN | qt.waylandcompositor.hardwareintegration] Failed to initialize EGL display. Could not get EglDisplay for window.

what is meaning of these errors?
Does it because there is something in Ubuntu desktop already using "org.freedesktop.Notifications", so we cannot use it?