private Session requireActiveSession(Tenant tenant, ApplicationId applicationId) {
        return getActiveSession(tenant, applicationId)
                .orElseThrow(() -> new IllegalArgumentException("Application '" + applicationId + "' has no active session."));
    }
Gets the active Session for the given application id.

@return the active session, or null if there is no active session for the given application id.
configserver/src/main/java/com/yahoo/vespa/config/server/ApplicationRepository.java