G_IS_DBUS_CONNECTION (connection)' failed

Status changed to 'Confirmed' because the bug affects multiple users.

I can confirm it occurs in Bionic. The fact that you have a machine that does not show this message is remarkable to me, as GTK apps tend to be really noisy, showing all sorts of bad looking messages when run from terminal. In fact, I would call that expected behavior. That said, especially given everything otherwise works fine, this is extremely low on the priority list.

@Walter Lapchynski
"Everything otherwise works fine"; surely not. For example, the installation program of hplip just crashed with "GLib-GIO-CRITICAL **: 16:14:04.867: g_dbus_proxy_new: assertion 'G_IS_DBUS_CONNECTION (connection)' failed" after the GUI dialog screen was launched and I pressed <Continue> (the error message was repeated three times and the installation program hanged up). I restarted the installation choosing "interactive" instead of "GUI" (which was the default choice) but I had to uninstall the previous failed installation, which had already written stuff on disk etc.
I would expect that any installation or script that launches a GUI from terminal would be affected.
At any rate, the bug makes for extremely bad publicity for Ubuntu and Linux. It's a CRITICAL error that keeps coming up; as if the aircraft's rocket engines sputtered and stuttered all the time but the crew tried to reassure you by saying, "don't worry, everything otherwise works fine". Some googling shows that quite many people are affected by the bug, both in Ubuntu and in other distributions.
In my opinion, this should be high on the priority list.

I meant everything works fine.... with leafpad. Note too that few users actually launch graphical applications from the command line, at which point the noisy errors are all but unnoticed.

If you want to make a new bug for hplip, that might make sense.

OK, I agree that leafpad works fine. However, the present bug is not leafpad specific, leafpad is not even mentioned in the bug description: "G_IS_DBUS_CONNECTION (connection)' failed ". I think that collecting the related bugs under this heading, instead of scattering them into several minor sections, would be more informative and effective. I'd say it makes sense ;)

No, it doesn't. If hplip shows it and fails and leafpad shows it and succeeds, you have two completely different situations.

Is this finally fixed ?

I confirm this on Lubuntu 18.04.3 kernel 5.0.0

Same problem with gEdit on Lubuntu 19.10 (kernel 5.3.0-24-generic)

(org.gnome.gedit:8480): GLib-GIO-CRITICAL **: 14:22:15.159: g_dbus_proxy_new: assertion 'G_IS_DBUS_CONNECTION (connection)' failed

But not present with leafpad 0.8.18.1 on Lubuntu 19.10.

I assume that leafpad issue is solved on Lubuntu 19,10.
I think gedit is not the default text editor on Lubuntu, even not included in the installation iso so i don't know if that could be an issue,

This is not a leafpad issue. For me, the problem still happens in Lubuntu 19.04, but it happens with the Vivaldi web browser. Maybe I'll upgrade to 19.10 this weekend to see if the problem goes away.

purge fcitx-module-dbus might help