Fatal error and Undefined index

I have found out that this notice appears because of the entity bundle filter in the view.
Strange thing is that other views that filter on a bundle, don't show the notice.

This issue now has turned into a fatal error:
Fatal error: Class name must be a valid object or a string in common.inc on line 7999

That line is: return $controllers[$entity_type];

/**
 * Gets the entity controller for an entity type.
 *
 * @return DrupalEntityControllerInterface
 *   The entity controller object for the specified entity type.
 */
function entity_get_controller($entity_type) {
  $controllers = &drupal_static(__FUNCTION__, array());
  if (!isset($controllers[$entity_type])) {
    $type_info = entity_get_info($entity_type);
    $class = $type_info['controller class'];
    $controllers[$entity_type] = new $class($entity_type);
  }
  return $controllers[$entity_type];
}

When I add a bundle filter to an entity view (i.e. "Project: project type, The type of this project entity") this fatal error shows up on a blank page.

I found the problem: it was caused by an entity reference field that was pointing at an entity bundle which was not part of the view.
A fatal error should not happen though.