Fix NPE when unregistering custom contribution to userManager extension point

2017-03-14 00:10:11,615 ERROR [localhost-startStop-2] [org.nuxeo.runtime.model.impl.RegistrationInfoImpl] Failed to unregister extension. Contributor: RegistrationInfo: service:pe.nuxeo.config.saml2 to service:org.nuxeo.ecm.platform.usermanager.UserService; xpoint: userManager
java.lang.NullPointerException
	at org.nuxeo.ecm.platform.usermanager.UserManagerImpl.setConfiguration(UserManagerImpl.java:231)
	at org.nuxeo.ecm.platform.usermanager.UserService.recomputeUserManager(UserService.java:104)
	at org.nuxeo.ecm.platform.usermanager.UserService.unregisterContribution(UserService.java:143)
	at org.nuxeo.runtime.model.DefaultComponent.unregisterExtension(DefaultComponent.java:58)
	at org.nuxeo.runtime.model.impl.ComponentInstanceImpl.unregisterExtension(ComponentInstanceImpl.java:217)
	at org.nuxeo.runtime.model.impl.ComponentManagerImpl.unregisterExtension(ComponentManagerImpl.java:285)
	at org.nuxeo.runtime.model.impl.RegistrationInfoImpl.deactivate(RegistrationInfoImpl.java:407)

        if (cacheService != null && descriptor.userCacheName != null) {
            principalCache = cacheService.getCache(descriptor.userCacheName);
            principalCache.invalidateAll();
        }