Avoid error messages.

#5 
When the field (in my case `pluggable_node_access`) had content and then the content was removed, during the submitting process, the entity arrive NULL.
I tried to avoid it in my site is several ways (unsetting `$form_state['values']['pluggable_node_access'][LANGUAGE_NONE][0]['target_id']` and parts of it) but nothing seems to help avoiding the error.