Non-fatal assertion failure when loading Xfburn without a burner

Steps

On both 0.6.2 and the latest sources:

1. Disconnect all burners
2. Launch Xfburn
3. Acknowledge the "no burners" warning
4. Observe a non-fatal assertion failure in the console

Output

DBG[xfburn-main.c:270] main(): xfburn version 0.6.3git-67388ba for Xfce 4.16

DBG[xfburn-settings.c:275] load_settings(): parsed
** Message: 17:37:36.781: Using UDEV
DBG[xfburn-device-list.c:229] xfburn_device_list_init(): Constructing device list
** Message: 17:37:38.460: Using gstreamer transcoder.

(xfburn:1165649): GLib-GIO-CRITICAL **: 17:37:38.491: g_simple_action_set_enabled: assertion 'G_IS_SIMPLE_ACTION (simple)' failed