Update user_node_access.module

Closes #3423743

user_node_access.module

@@ -40,11 +40,14 @@ function user_node_access_form_alter(&$form, FormStateInterface $form_state, $na
       '#group' => 'advanced',
     ];
     $defaultUsers = [];
-    if ((unserialize(@$values['user'])) == true) {
+    if (!empty($values['user']) && is_string($values['user'])) {
+      $unserialized_data = unserialize(@$values['user']);
+      if ($unserialized_data !== false) {
       foreach (unserialize($values['user']) as $k => $val) {
         $defaultUsers[] = User::load($val['target_id']);
       }
     }
+  }
     $form['user_node_access']['user'] = [
       '#type' => 'entity_autocomplete',
       '#target_type' => 'user',