Liferay ServiceContext in Vaadin Portlet

Hi guys,
I developed a vaadin portlet and i deployed to liferay.
In this portlet I have to create liferay user and i used :

User operatore = UserLocalServiceUtil.addUser(all parameters…,serviceContext);

How get ServiceContext from liferay into Vaadin Portlet ?
I tried with :
PortletRequest portleRequest = ((VaadinPortletRequest)vaadinrequest).getPortletRequest();
ServiceContext serviceContext = ServiceContextFactory.getInstance(portleRequest);

but i have Nullpointer exception on this row “ServiceContext serviceContext = ServiceContextFactory.getInstance(portleRequest);”

portleRequest is not null

Can you help me ??
Thank you.