Node entity type not within the array ore any other reason. See detailed error output.

Error message:
Notice: Undefined index: node in _custom_formatters_preview_variables() (line 231 of /home/frizi/Desktop/www/christ/sites/all/modules/contrib/custom_formatters/plugins/export_ui/custom_formatters.inc).
Warning: array_keys() expects parameter 1 to be array, null given in _custom_formatters_preview_variables() (line 231 of /home/frizi/Desktop/www/christ/sites/all/modules/contrib/custom_formatters/plugins/export_ui/custom_formatters.inc).

Solution:
if(array_key_exists($entity_type, $result)){
$entities = entity_load($entity_type, array_keys($result[$entity_type])); // FLE
}
else { $entities = array(); }