Notice: Undefined index: und in views_handler_field_field->get_value() (line 740 of views_handler_field_field.inc)

I think this is a duplicate of #1245244: Undefined index: delta in views_handler_field_field->get_value(), which adresses 'delta' instead of 'und'.
Error appears when looping at display data when some columns/datacells have no data, in e.g. mymodule_views_plugin_style_xyz.inc :

foreach ($records as $row_index => $row) {
  foreach ($this->view->field as $key => $field) {
    if (!$field->options['exclude']) {
      $object_value = $this->view->field[$key]->get_value($row);
    }
  }
}

The line 740 in views_handler_field_field::getvalue() may return an empty array, which is incorrectly tested for in line 686:

    // The field we are trying to display doesn't exist on this entity.
+    if (empty($entity->{$this->definition['field_name']})) {
-    if (!isset($entity->{$this->definition['field_name']})) {
      return array();
    }
...
    if ($field == 'entity') {
      return $entity;
    }
    else {
      return $entity->{$this->definition['field_name']}[$langcode];
    }
  }

Patch coming up.