#vulnerable code 
                                public synchronized void stop()
    {
        if (!started)
        {
            logger.error(this.callContext + " Already stopped !");
            return;
        }

        started = false;

        JigasiBundleActivator.osgiContext.removeServiceListener(this);

        if (telephony != null)
        {
            telephony.removeCallListener(callListener);
            telephony = null;
        }

        if (muteIqHandler != null)
        {
            // we need to remove it from the connection, or we break some Smack
            // weak references map where the key is connection and the value
            // holds a connection and we leak connection/conferences.
            getConnection().unregisterIQRequestHandler(muteIqHandler);
        }

        gatewaySession.onJvbConferenceWillStop(this, endReasonCode, endReason);

        leaveConferenceRoom();

        if (jvbCall != null)
        {
            CallManager.hangupCall(jvbCall, true);
        }

        if (xmppProvider != null)
        {
            xmppProvider.removeRegistrationStateChangeListener(this);

            // in case we were not able to create jvb call, unit tests case
            if (jvbCall == null)
            {
                logger.info(
                    callContext + " Removing account " + xmppAccount);

                xmppProviderFactory.unloadAccount(xmppAccount);
            }

            xmppProviderFactory = null;

            xmppAccount = null;

            xmppProvider = null;
        }

        gatewaySession.onJvbConferenceStopped(this, endReasonCode, endReason);

        setJvbCall(null);
    } 
                            #location 24 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                public synchronized void stop()
    {
        if (!started)
        {
            logger.error(this.callContext + " Already stopped !");
            return;
        }

        started = false;

        JigasiBundleActivator.osgiContext.removeServiceListener(this);

        if (telephony != null)
        {
            telephony.removeCallListener(callListener);
            telephony = null;
        }

        if (muteIqHandler != null)
        {
            // we need to remove it from the connection, or we break some Smack
            // weak references map where the key is connection and the value
            // holds a connection and we leak connection/conferences.
            XMPPConnection connection = getConnection();
            if (connection != null)
            {
                connection.unregisterIQRequestHandler(muteIqHandler);
            }
        }

        gatewaySession.onJvbConferenceWillStop(this, endReasonCode, endReason);

        leaveConferenceRoom();

        if (jvbCall != null)
        {
            CallManager.hangupCall(jvbCall, true);
        }

        if (xmppProvider != null)
        {
            xmppProvider.removeRegistrationStateChangeListener(this);

            // in case we were not able to create jvb call, unit tests case
            if (jvbCall == null)
            {
                logger.info(
                    callContext + " Removing account " + xmppAccount);

                xmppProviderFactory.unloadAccount(xmppAccount);
            }

            xmppProviderFactory = null;

            xmppAccount = null;

            xmppProvider = null;
        }

        gatewaySession.onJvbConferenceStopped(this, endReasonCode, endReason);

        setJvbCall(null);
    }