I can't retrieve UserManager service from WebEngine

Hi all,

I'm modifying the sample4 and I want to retrieve the UserManager  
service. I've noticed that admin module uses the following piece of  
code:

def userManager = Framework.getService(UserManager.class);

But it doesn't work for me. It returns a null value.

I've imported the following libraries:

import org.nuxeo.runtime.api.*;
import org.nuxeo.ecm.platform.usermanager.*;

What am I doing wrong?

Thanks in advance.

Best regards.