fix crash

sometime i have this bug, 
14:42:37.691 [error] Error in controller error undef [{auth_view_lib_tags,translated_blocks,[],[]},{boss_lang,'-extract_view_strings/1-fun-1-',2,[{file,"src/boss/boss_lang.erl"},{line,163}]},{lists,foldl,3,[{file,"lists.erl"},{line,1248}]},{boss_lang,extract_strings,1,[{file,"src/boss/boss_lang.erl"},{line,106}]},{boss_lang,extract_strings,2,[{file,"src/boss/boss_lang.erl"},{line,111}]},{cb_admin_lang_controller,show,4,