Adding back the nodes in xml using Java JAXB

Question

The following is my xml:

<pickingOrderBeginEventMessage xmlns="http://www.xmlns.walmartstores.com/SuppyChain/FulfillmentManagement">
    <hdr:MessageHeader>
    </hdr:MessageHeader>
    <MessageBody>
        <orderBeginEvent>
            <node>
            </node>
            <fulfillmentOrders>
                <fulfillmentOrder>
                    <orderNbr>1</orderNbr>
                    <priorityCode>1</priorityCode>
                    <assignedPickUserId>alpha</assignedPickUserId>
                    <userId>123</userId>
                </fulfillmentOrder>
            </fulfillmentOrders>
        </orderBeginEvent>
        <MessageExtensions />
    </MessageBody>
</pickingOrderBeginEventMessage>

I want to remove the value <orderBeginEvent> and add it back later.I can achieve the first part where I am deleting but cant set it back to the way it was in the xml. It is throwing me a nullpointerexception.

This is my POJO class.

    public class MessageBody implements Serializable {

    private static final long serialVersionUID = 1L;

    @XmlElement(name = "RoutingInfo")
    RoutingInfoPickBgn routingInfo;
    @XmlElement(name = "orderBeginEvent")
    @Accessors(fluent = true)
    OrderBeginEvent orderBeginEvent;
}

This is how I am marshaling it using JAXB:

 public String updateXML() {
        try {
            JAXBContext context = JAXBContext.newInstance(PickingOrderBeginEventMessage.class);
            Marshaller mar = context.createMarshaller();
            mar.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, Boolean.TRUE);
            File updateXml = new File("src/test/resources/xml/updatePickBgn.xml");
            mar.marshal(pickBegin, updateXml);
            String absolutePath = updateXml.getAbsolutePath();
            String updatedXml = readFromResources(absolutePath);
            return updatedXml;
        } catch (Exception ex) {
            ex.printStackTrace();
            return null;
        }
    }

I am able to remove orderBeginEvent by setting it to null but I am not sure how to add it back.

  public void checkNoOrderBeginEvent() {

        FulfillmentOrder orderNbr = pickBegin.getMessageBody().setOrderBeginEvent(null);
        String updatedXml = updateXML();
    //This code removed the OrderBeginEvent

}

The objective of the code is to after deleting the orderBeginEvent i need to put it back the way it was. I can achieve the former which is to delete the orderBeginEvent but dont know how to set it back. Any help will be appreciated.