null pointer on toString

Hello

I have created a custom packet

public XmlStringBuilder toXML() {

        XmlStringBuilder buf = new XmlStringBuilder();

        buf.halfOpenElement(ELEMENT);

        buf.optAttribute("to", getTo());

        buf.optAttribute("from", getFrom());

        buf.attribute("t", type);

        buf.attribute("sid",ackedforid);

        buf.attribute("time", String.valueOf(acktime));

        buf.attribute("q", String.valueOf(quick));

        buf.rightAngleBracket();

        buf.append(getExtensionsXML());

        // Add the error sub-packet, if there is one.

        appendErrorIfExists(buf);

        buf.closeElement(ELEMENT);

        return buf;

    }

public static Ack parseAck(XmlPullParser parser)

            throws Exception {

        ParserUtils.assertAtStartTag(parser);

        final int initialDepth = parser.getDepth();

        Ack.Type type = Ack.Type.mra;

        String typeString = parser.getAttributeValue("", "t");

        if (typeString != null && !typeString.equals("")) {

            type = Ack.Type.fromString(typeString);

        }

        Ack ack = new Ack(type);

        ack.setTo(ParserUtils.getJidAttribute(parser, "to"));

        ack.setFrom(ParserUtils.getJidAttribute(parser, "from"));

        ack.setStanzaId(parser.getAttributeValue("", "id"));

        ack.setAckedforid(parser.getAttributeValue("","sid"));

        ack.setAcktime(Integer.parseInt(parser.getAttributeValue("","time")));

        ack.setQuick(Boolean.parseBoolean(parser.getAttributeValue("","q")));

        return ack;

    }

When the server send response as

<Ack from='dummy.com' to='xxxxx@server/resoucerce:' t='sa' id='6177764630472404992-3' time='1438372915' q='0'/>

response.toXml().toString fails with null pointer at

java.lang.NullPointerException: Attempt to invoke interface method 'int java.lang.CharSequence.length()' on a null object reference

            at xmpp.util.LazyStringBuilder.length(LazyStringBuilder.java:73)

            at xmpp.util.LazyStringBuilder.toString(LazyStringBuilder.java:101)

            at xmpp.util.XmlStringBuilder.toString(XmlStringBuilder.java:455)

            at xmpp.packet.Stanza.toString(Stanza.java:471)

            at com.griffin.messenger.ConnectionsManager.tcpConnectionReceivedData(ConnectionsM anager.java:1622)

What wrong am i doing