Problems with p: ajax in primefaces

Good morning, friends! I am a beginner in Java and I am trying to develop a system for church management. After many attempts, I can make ajax work, however I am not able to retrieve the id of the state that was selected in selectOneMenu. The id returned is always null. Here are snippets of code:

<h:form id="formMembroCadastro">
    <h:panelGrid id="panel2" columns="4">
        <p:outputLabel for="estado" value="Estado: " 
                   style="margin-left: 30px" />
        <p:selectOneMenu id="estado" value="#
                     {membroBean.membro.endereco.bairro.cidade.estado}"
                     readonly="#{membroBean.acao == 'Consultar'}">
            <p:ajax listener="#{membroBean.getCidadesDoBanco}"
                    immediate="true" update=":formMembroCadastro:cidade" />
            <f:selectItem itemValue="" itemLabel="Escolha o estado"
                          noSelectionOption="true" />
            <f:selectItems value="#{membroBean.estados}" var="estado"
                           itemValue="#{estado.id}" itemLabel="#
                           {estado.nome}" />
        </p:selectOneMenu>
    </h:panelGrid>
</h:form>

I have already changed several things, like @ViewScoped, @SessionScoped, event, listener, update.

@ManagedBean
@ViewScoped
public class MembroBean implements Serializable {

    private static final long serialVersionUID = 1L;

    private Membro membro = new Membro();
    private Estado estado;

    public void getCidadesDoBanco() {
        System.out.println(estado.getId());
        cidades = new DAO<Cidade>(Cidade.class).listaGeral();
    }
}

When I put

System.out.println(Estado.class);

prints "class br.com .modelo.Estado".

Thank you in advance for your help.