I know it is mainly cosmetic but could this be rewritten as

``` php
if (!class_exists('Symfony\Component\ExpressionEngine\ExpressionEngine')) {
    throw new RuntimeException('Unable to use expressions as the Symfony ExpressionEngine component is not installed.');
}

if (null === $this->expressionEngine) {
    $this->expressionEngine = new ExpressionEngine();
}

return $this->expressionEngine;
}
```