Error: EntityManager is closed

My command system, when trying to merge tables, I use EntityManager.merge, however the following error occurs: EntityManager is closed.

Here is one of the functions in my code that causes this error:

public boolean trocar_Mesa(BigDecimal origem, BigDecimal destino,String usuario) {
    ok = true;
    mensagem = "";

    Tab_Log_Troca_Mesa oMesa = null;
    Tab_Mesas oMesaOri = null;
    Tab_Mesas oMesaDest = null;
    Calendar hoje = Calendar.getInstance();
    DateTime agora = new DateTime();

    EntityManager em = JPAUtil.getEntityManager();

    try {
        em.getTransaction().begin();

        for (Tab_Comanda oComanda : (new Tab_ComandaDao())
                .listar_ComandasMesas(origem)) {
            oComanda.setNumero_Mesa(destino);
            em.merge(oComanda);
        }

public List<Tab_Comanda> listar_ComandasMesas(BigDecimal mesa)
{
    List<Tab_Comanda> oLista = null;
    ok = true;
    mensagem = "";

    String sql = 
        "select * from TAB_COMANDAS where NUMERO_MESA = :pNumeroMesa";
    EntityManager em = JPAUtil.getEntityManager();;

    try
    {
        Query query = em.createNativeQuery(sql, Tab_Comanda.class);
        query.setParameter("pNumeroMesa", mesa);
        oLista = query.getResultList();
    }
    catch (Exception ex)
    {
        ex.printStackTrace();
        ok = false;
        mensagem = ex.getMessage();
    }
    finally{
        if (em.isOpen()){
            em.close();
        }
    }
    return oLista;
}

I did not find any forum that could help me and I'm starting now in JavaWEB, has anyone ever encountered this problem and / or know how to solve it?

Thank you in advance.