Exception in URL Factory initialization

When multiple thread are calling the xwiki URL factory service creation it's possible that one thread will throw a NPE:

java.lang.NullPointerException
	at com.xpn.xwiki.web.XWikiURLFactoryServiceImpl.createURLFactory(XWikiURLFactoryServiceImpl.java:81)
	at com.xpn.xwiki.web.XWikiAction.execute(XWikiAction.java:108)
	at org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:431)
	at org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:236)
	at org.apache.struts.action.ActionServlet.process(ActionServlet.java:1196)
	at org.apache.struts.action.ActionServlet.doGet(ActionServlet.java:414)

The reason is because the code does the following check:

    public XWikiURLFactoryService getURLFactoryService()
    {
        if (urlFactoryService == null) {
            synchronized (URLFACTORY_SERVICE_LOCK) {
                if (urlFactoryService == null) {

but later the init is done is 2 steps: the creation of the object and its initilization:

urlFactoryService =
(XWikiURLFactoryService) Class.forName(urlFactoryServiceClass).newInstance();
urlFactoryService.init(this);
code}

Thus it's possible that urlFactoryService isn't null but still isn't initialized.