[fix] Accessing undefined elements on Configuration > Text filters

Issue tracker:
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=33552

Global configuration shows some notices on text filters tab when error reporting is set to maximum.

See image:
http://joomlacode.org/gf/download/trackeritem/33552/158508/undefined-text-filters.png