If identifier is left null in the widget, the following happens:
[Thu Apr 30 18:28:04 2015] [error] [client 107.2.152.202] PHP Warning: in_array() expects parameter 2 to be array, null given in /nas/wp/www/cluster-10671/largoproject/wp-content/plugins/dfp/dfw-widget.php on line 99, referer: http://npq.largoproject.wpengine.com/wp-admin/widgets.php
If identifier is left null in the widget, the following happens: