Running xfce4-panel from the terminal, I get the following error message:
(wrapper-2.0:7138): Gtk-CRITICAL **: gtk_container_foreach: assertion 'GTK_IS_CONTAINER (container)' failed
So this is either a regression in xfce4-indicator-plugin or in the xfce4-panel's wrapper2.
Running xfce4-panel from the terminal, I get the following error message: foreach: assertion 'GTK_IS_CONTAINER (container)' failed
(wrapper-2.0:7138): Gtk-CRITICAL **: gtk_container_
So this is either a regression in xfce4-indicator -plugin or in the xfce4-panel's wrapper2.