assertion 'window->update_and_descendants_freeze_count > 0' failed for libappindicator3 application in notification area
I have a problem with an application that tries to add widgets in the notification area, namely multiload-ng-indicator (freshly compiled from latest source from https://github.com/udda/multiload-ng). It does not manage to display correctly the different monitored items; I only see the default app icon (a pulse over a green circle). You'd say it's the application's fault but I experience the same error with others (like indicator hardware). So I guess it has something to do with libappindicator3 which is the common factor between these problematic apps.
Running the applications in a terminal I get a Gdk critical messages: Gdk-CRITICAL **: 14:53:40.178: gdk_window_thaw_toplevel_updates: assertion 'window->update_and_descendants_freeze_count > 0' failed (Followed by application warnings: Indicator is not connected to panel, thus it cannot be displayed.)
I am running Xfce 4.14, specifically xfce4-panel 4.14.4 (from CentOS 8 repository).
Any idea what's wrong? Any help would be appreciated.