bug fixe : error message

Signed-off-by: bachy <git@g-u-i.net>

handlers/views_handler_field.inc

@@ -372,7 +372,7 @@ class views_handler_field extends views_handler {
    *   Optional name of the field where the value is stored.
    */
   function get_value($values, $field = NULL) {
-    $alias = isset($field) ? $this->aliases[$field] : $this->field_alias;
+    $alias = isset($field) && isset($this->aliases[$field]) ? $this->aliases[$field] : $this->field_alias;
     if (isset($values->{$alias})) {
       return $values->{$alias};
     }