I'd suggest checking if the object is an instance of `Exception` instead.  This also allows you to retrieve the error code from the `Exception` object and to me is a more practical solution:

```php
if ($error instanceof Exception) {
    $code = $error->getCode();
} else {
    $code = $error->get('code');
}

$document->setTitle(JText::_('Error') . ': ' . $code);
```