Comment 4 for bug 191725

Thanks to Sjoerd Simons who figured the issue

addon-dell-backlight.cpp filter_function() is using

 AC = libhal_device_get_property_bool (halctx,
          "/org/freedesktop/Hal/devices/acpi_AC",
          "ac_adapter.present",
          &err);

or since the sysfs changes there is no "ac_adapter.present" key there and the same err variable is used when calling dbus_message_get_args later