private void verifySessionId(Serializable sessionId) {
        if (sessionId == null) {
            String msg = "sessionId returned from doCreate implementation is null.  Please verify the implementation.";
            throw new IllegalStateException(msg);
        }
    }
Ensures the sessionId returned from the subclass implementation of {@link #doCreate} is not null and not
already in use.

@param sessionId session id returned from the subclass implementation of {@link #doCreate}
core/src/main/java/org/apache/shiro/session/mgt/eis/AbstractSessionDAO.java