Fixed Notice error

entitytemplates.php

@@ -245,7 +245,7 @@ function entitytemplates_civicrm_pre($op, $objectName, $id, &$params) {
  *
  */
 function entitytemplates_civicrm_validateForm($formName, &$fields, &$files, &$form, &$errors) {
-  if ($form->_entityTemplate) {
+  if (property_exists($form, '_entityTemplate') && $form->_entityTemplate) {
     if (!empty($fields['entity_template_title'])) {
       $params = [
         'title' => $fields['entity_template_title'],