SetWrappedData for DataModel isn't working properly

Question

I have a form with a panelGrid, where the user puts in some data:

<h:form>
    <p:panelGrid var="workOrder" value="#{WorkOrder}" columns="2"
        headerClass="header" footerClass="footer">
        <p:outputLabel value="#{msg.project_number}: " for="projectNr" />
        <p:column>
            <p:inputText id="projectNr"
                value="#{mbWorkOrderController.workOrderCurrent.projectNr}"
                required="true" requiredMessage="#{msg.input_missing}"
                maxlength="6" />
        </p:column>
        <p:outputLabel value="#{msg.date}: " for="date" />
        <p:column>
            <p:calendar id="date"
                value="#{mbWorkOrderController.workOrderCurrent.date}"
                mode="popup" navigator="true" showOn="button"
                pattern="dd.MM.yyy" />
        </p:column>
    </p:panelGrid>
    <p:commandButton action="#{mbWorkOrderController.saveWorkOrder()}"
        value="#{msg.save}" style="margin-top: 10px" />
    <p:commandButton
        action="#{mbWorkOrderController.cancelWorkOrder()}"
        value="#{msg.cancel}" immediate="true" style="margin-top: 10px" />
</h:form>

When the user clicks "save", the method saveWorkOrder() is called. Inside, I am trying to use the method setWrappedData() on a DataModel variable, after I create the result using the EntityManagerss createNamedQuery():

public String saveWorkOrder() {
    try {
        utx.begin();
        workOrderCurrent = em.merge(workOrderCurrent);
        em.persist(workOrderCurrent);
        List<WorkOrder> resultList = em.createNamedQuery("SelectWorkOrders").getResultList();
        workOrdersList.setWrappedData(resultList);
        utx.commit();
    } catch (NotSupportedException e) {
        e.printStackTrace();
    } catch (SystemException e) {
        e.printStackTrace();
    } catch (SecurityException e) {
        e.printStackTrace();
    } catch (IllegalStateException e) {
        e.printStackTrace();
    } catch (RollbackException e) {
        e.printStackTrace();
    } catch (HeuristicMixedException e) {
        e.printStackTrace();
    } catch (HeuristicRollbackException e) {
        e.printStackTrace();
    }
    return "rainErosion";
}

Unfortunately I am getting a NullPointerException in setWrappedData(resultList) although resultList contains one element (with the actual user input) and is not pointing to Null, so normaly this shouldn't happen.

Does anyone know, why this happens?