The Custom Liferay service function can not open session ()

This problem has been frustrating me for quite some time and I'm hoping someone can help me out here. I am using Service Builder to expose a custom entity to the JSON Web Service API, which I want to use in my portlet. I cannot use Dynamic Queries, and since there will be several more complicated queries later on with multiple joins, I feel custom sql is the best option. However, I'm not even able to begin the query because the call to openSession() throws a NPE. Here's my code (I apologize for the length, but I really have no idea what I'm doing wrong here and I'm just trying to include everything that's relevant):

ServiceImpl class:

@JSONWebService
public class MBMessagesAsDiscussionPrimeServiceImpl extends MBMessagesAsDiscussionPrimeServiceBaseImpl {
    @Override
    public List<MBMessagesAsDiscussionPrime> getMessagesAsDiscussionPrime() throws SystemException {
        MBMessagesAsDiscussionPrimeFinder finder = new MBMessagesAsDiscussionPrimeFinderImpl();
        return finder.findByGroupId();
    }
}

My FinderImpl class:

public class MBMessagesAsDiscussionPrimeFinderImpl extends BasePersistenceImpl<MBMessagesAsDiscussionPrime> implements MBMessagesAsDiscussionPrimeFinder {

    @Override
    public List<MBMessagesAsDiscussionPrime> findByGroupId() throws SystemException {
        SessionFactory sessionFactory = (SessionFactory) PortalBeanLocatorUtil.locate("liferaySessionFactory");

        Session session = null;
        try {
            session = sessionFactory.openSession(); //exception here

            //other stuff here, eventually...
        } catch (Exception e) {
            throw new SystemException(e);
        } finally {
            closeSession(session); //throws NPE here
        }
    }
}

custom query:

<?xml version="1.0" encoding="UTF-8"?>
<custom-sql>
    <sql
        id="com.test.portlet.service.persistence.MBMessagesAsDiscussionPrimeFinder.findByGroupId">
        <![CDATA[
            SELECT * FROM MBMessage, MBThread
            WHERE
                (MBMessage.threadId = MBThread.threadId) AND
                (MBThread.groupID = ?)
            ORDER BY
                MBThread.rootMessageId DESC, MBMessage.messageId ASC
        ]]>
    </sql>
</custom-sql>

and service.xml:

<?xml version="1.0"?>
<!DOCTYPE service-builder PUBLIC
"-//Liferay//DTD Service Builder 6.1.0//EN"
"http://www.liferay.com/dtd/liferay-service-builder_6_1_0.dtd">

<service-builder package-path="com.test.portlet">
    <namespace>MBMessagesAsDiscussionPrime</namespace>

    <entity name="MBMessagesAsDiscussionPrime" uuid="true" local-service="true" remote-service="true">

        <column name="messageId" type="long" primary="true" />
        <column name="threadId" type="long"/>
        <column name="userId" type="long"/>
        <column name="userName" type="String"/>
        <column name="body" type="String"/>

        <reference package-path="com.liferay.portlet.messageboards" entity="MBMessage" />
        <reference package-path="com.liferay.portlet.messageboards" entity="MBThread" />
    </entity>
</service-builder>

The service function is visible from localhost:8080/custom-query-portlet/api/jsonws, which is where I'm calling it from. Is there something in particular I need to do since this is being called remotely, besides setting @JSONWebService on the ServiceImpl class? Please, someone help me out on this. It's driving me up a wall!

Try this code

 @Override
    public List<MBMessagesAsDiscussionPrime> findByGroupId() throws SystemException {
        Session session = null;
        try {
            session = openSession();

            //other stuff here, eventually...
        } catch (Exception e) {
            throw new SystemException(e);
        }
    }