When restarting the datetime plugin, there is a GLib assertion failure.

Reproduce as follows:
Add one instance of the plugin to the panel with the default configuration.
From a terminal window exit, start, then restart the panel.
xfce4-panel -x
xfce4-panel &
xfce4-panel -r

$ xfce4-panel -r
(xfce4-panel:1774): GLib-CRITICAL **: g_source_remove: assertion `tag > 0' failed
** Message: Restarting xfce4-panel...
DBG[datetime.c:670] datetime_new(): Starting datetime panel plugin

On subsequent trials, "xfce4-panel -x" produces this message too.

Revision is 4921.