For some properties, the launcher and systray plugins use a GPtrArray.
In their _get_property, they set the array to the value using g_value_set_boxed,
followed by a call to xfconf_array_free, which calls g_value_unset, g_free on each
array's values and at the end a call to g_ptr_array_free.

This results in xfconf_channel_set_property getting an array with length 0, making
the preparation of the GVariant fails (xfconf_gvalue_to_gvariant).

This is fixed in both plugins, by using a g_ptr_array_new_full, setting the boxed value
and releasing the reference count of the array, the array's values are later destroyed
by the (GDestroyNotify) function. No idea why it worked before with xfconf < 4.13.0.