public function containsNull()
    {
        foreach (array_keys($this->getDataPlaceHolder()) as $key) {
            $v = $this[$key];
            if ($v instanceof Container && $v->isNull()) {
                return true;
            } elseif (($v instanceof QtiString && $v->getValue() === '') || is_null($v)) {
                return true;
            }
        }

        return false;
    }