Zenity works for many commands but not for notification

[stephane.plaisance@…]

ElCapitan with all ports up to date

I installed xinit and xorg-server as advertised in ​https://trac.macports.org/ticket/42739 and rebooted but the following problem remains.

--notification commands do not show a zenity dialog and report an error

zenity --notification --text "show me something."

** (zenity:2501): WARNING **: Error showing notification: The connection is closed

by contrast (but with a warning), I can run

zenity --info --text "show me something."
Gtk-Message: GtkDialog mapped without a transient parent. This is discouraged.

Thanks for any hint to improve this