Selected item in p: selectOneMenu gets null in listener

I have a p: selectOneMenu, with data coming from the database, inside a p: dialog, along with a p: messages and other fields. I want to execute an action (Display some messages) when selecting certain option in p: selectOneMenu, but I get a nullPointerException because the value gets null in the p: ajax listener method. I have already tried to use onchange="submit();" in p: selectOneMenu, the nullPointerException for, but the method simply does not execute and no exception is shown, simply nothing happens.

Dialog

<p:dialog id="dialogRelato"
                  showEffect="fade" hideEffect="fade"
                  modal="true" header="Novo Relato"
                  widgetVar="dialogRelato" minHeight="40"
                  resizable="false">

            <p:messages id="menssagens" autoUpdate="true" closable="true" />

                <b>
                    <h:outputLabel value="Tipo de Relato: " />
                </b>
                <p:selectOneMenu style="height: 20px; width: 200px;"
                                 value="#{livroOrdemController.tipoRelatoSelecionado}"
                                 converter="tipoRelatoConverter">
                    <f:selectItems value="#{livroOrdemController.preencherComboTiposRelatos()}"/>
                    <p:ajax event="change" listener="#{livroOrdemController.onSelectItemMenuChange()}" process="@this" />
                </p:selectOneMenu>
                <br />
                <br />
            <p:calendar id="calendarDataOcorrencia"
                            pattern="dd/MM/yyyy"
                            locale="pt_BR"
                            showOn="button"
                            value="#{livroOrdemController.dataOcorrencia}">
                    <p:ajax event="dateSelect" listener="#{livroOrdemController.onDataOcorrenciaSelect()}"/>
                </p:calendar>
  </p:dialog>

Method not Managed Bean

public void onSelectItemMenuChange() {
   if(tipoRelatoSelecionado.getDescricao().equals("Acidentes e Danos")) {
       FacesUtils.mensErro("Teste Listener");
   }
}

Converter

@FacesConverter(value="tipoRelatoConverter")
public class TipoRelatoConverter implements Converter {

public TipoRelatoConverter() {
}

@Override
public Object getAsObject(FacesContext context, UIComponent component, String string) {
    if (string == null || string.equals("Selecione...")) {
        return null;
    }

    Long id = Long.parseLong(string);
    TipoRelato tipoRelato = new TipoRelato();
    tipoRelato.setIdTipoRelato(id);

    return tipoRelato;
}

@Override
public String getAsString(FacesContext context, UIComponent component, Object object) {
    if (object == null) {
        return null;
    }

    if(object instanceof TipoRelato) {
        TipoRelato tipoRelato = (TipoRelato) object;

        return "" + tipoRelato.getIdTipoRelato();
    } 

    else {
        throw new IllegalArgumentException("object:" + object + "of type:" + 
                object.getClass().getName() + "; expected type:br.org.web.entities.TipoRelato");
    }
}

}