p:ajax change event request returns 500 error

Question

I am trying to set up 2 chained select boxes using JSF and PrimeFaces. I have created them as in the example on the official site, however:

1. When i enter the page using the new button(no existing record to be displayed) the change event will return a 500 error from the controller
2. When i enter the page using the edit button(there is an existing record) the change event will not return a 500 error, however the id from the select is not mapped to the record.

Any help would be appreciated.

Relevant code bits below:

<?xml version="1.0" encoding="UTF-8"?>
<ui:composition xmlns="http://www.w3.org/1999/xhtml"
            xmlns:ui="http://java.sun.com/jsf/facelets"
            xmlns:f="http://java.sun.com/jsf/core"
            xmlns:h="http://java.sun.com/jsf/html"
            xmlns:p="http://primefaces.org/ui"
            xmlns:o="http://omnifaces.org/ui"
            template="/WEB-INF/templates/page.xhtml">

<f:metadata>
    <o:viewParam name="userUploadedWorkId" value="#{userUploadedWorkController.userUploadedWork}"
                 converter="#{userUploadedWorkConverter}"
                 converterMessage="Solicitare eronata. Va rugam sa navigati in aceasta pagina dintr-o lista de lucrari"/>
</f:metadata>

<ui:define name="bread-crumb">
    <p:menuitem value="Lucrarile mele" outcome="user-uploaded-work-list?faces-redirect=true"/>
    <p:menuitem
            value="#{userUploadedWorkController.userUploadedWork.id eq null ? 'Lucrare noua' : userUploadedWorkController.userUploadedWork.title}"
            url="#"/>
</ui:define>

<ui:define name="page-content">

    <h:form>
        <p:panelGrid columns="2" columnClasses="right-aligned,raw" id="user-uploaded-work-details">
            <f:facet name="header">Detalii lucrare</f:facet>

            <p:outputLabel value="Titlu" for="uuwTitle"/>
            <p:inputText id="uuwTitle" value="#{userUploadedWorkController.userUploadedWork.title}" required="true"
                         style="width: 400px"/>


            <p:outputLabel value="Capitol" for="chapter"/>
            <p:selectOneMenu style="width: 400px" required="true" id="chapter" value="#{userUploadedWorkController.userUploadedWork.criterion eq null?'':userUploadedWorkController.userUploadedWork.criterion.chapter.id}" effect="fade">  
                <f:selectItem itemLabel="Select One" itemValue="" />  
                <f:selectItems value="#{chapters}" var="ch" itemLabel="#{ch.name}" itemValue="#{ch.id}"/> 
                <p:ajax update="uuwCriterion" listener="#{userUploadedWorkController.chapterChange}"/> 
            </p:selectOneMenu>            

            <p:outputLabel value="Criteriu" for="uuwCriterion"/>
            <p:selectOneMenu style="width: 400px" required="true" id="uuwCriterion" value="#{userUploadedWorkController.userUploadedWork.criterion eq null?'':userUploadedWorkController.userUploadedWork.criterion.id}" effect="fade">  
                <f:selectItem itemLabel="Select One" itemValue="" />  
                <f:selectItems value="#{userUploadedWorkController.criteria}" var="criterion" itemLabel="#{criterion.name}" itemValue="#{criterion.id}"/>  
            </p:selectOneMenu>     


            <f:facet name="footer">
                <p:message for="uuwTitle"/>
                <p:message for="uuwCriterion"/>
                <p:message for="uuwChapter"/>

                <p:commandButton action="#{userUploadedWorkController.save}" value="Salveaza atribute" process="@form"
                                 update="@form :messages" icon="ui-icon-disk"/>
                <p:button outcome="user-uploaded-work-list?faces-redirect=true" value="Lista lucrari"
                          icon="ui-icon-arrowreturnthick-1-e"/>
            </f:facet>
        </p:panelGrid>

    </h:form>

</ui:define>

The managed bean:

@Named
@ViewScoped
public class UserUploadedWorkController implements Serializable {
private static final long serialVersionUID = -4736897416993974840L;

@Inject
private OrganizationalChartService service;

private UserUploadedWork userUploadedWork = new UserUploadedWork();

private List<Criterion> criteria;

public UserUploadedWork getUserUploadedWork() {
    return userUploadedWork;
}

public void setUserUploadedWork(UserUploadedWork userUploadedWork) {
    this.userUploadedWork = userUploadedWork;
}

@PostConstruct
public void init() {
    criteria = service.findAllCriteria();
}

public List<Criterion> getCriteria() {
    return criteria;
}

public void setCriteria(List<Criterion> criteria) {
    this.criteria = criteria;
}

public String save() {
    userUploadedWork.setCriterion(service.findById(Criterion.class, userUploadedWork.getCriterion().getId()));
    userUploadedWork.setUser(service.findBySimpleProperty(User.class, "email", LoginController.getRequest().getUserPrincipal().getName()));
    userUploadedWork = service.merge(userUploadedWork);
    Messages.addFlashGlobalInfo("Salvare efectuata cu succes");
    return "user-uploaded-work-list?faces-redirect=true";
}


public void chapterChange() {
    if (userUploadedWork.getChapter() != null
            && userUploadedWork.getChapter().getId() != null) {
        criteria = service.findCriteriaByChapter(userUploadedWork.getChapter().getId());
    } else {
        criteria = new ArrayList<Criterion>();
    }
}
}