Hibernate NullPointerException when executing session.close ();

Very good, I am using hibernate for the first time and I am trying to make a simple application where I now skip an error that I do not know what it might mean. I followed a series of tutorials in which I explained how to configure everything and so far everything went well, I was able to execute selects, inserts and deletes without problems, but from a moment here I started to give problems with any operation I would like to execute .

hibernate.cfg.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-configuration PUBLIC "-//Hibernate/Hibernate Configuration DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
  <session-factory>
    <property name="hibernate.connection.driver_class">com.microsoft.sqlserver.jdbc.SQLServerDriver</property>
    <property name="hibernate.connection.url">jdbc:sqlserver://DESKTOP-A1M7E3R:1433;databaseName=Agencia</property>
    <property name="hibernate.connection.username">sa</property>
    <property name="hibernate.connection.password">1234</property>
    <property name="show_sql">true</property>
    <mapping resource="Models/Avion.hbm.xml"/>
    <mapping resource="Models/Sucursal.hbm.xml"/>
    <mapping resource="Models/Vuelo.hbm.xml"/>
    <mapping resource="Models/Cliente.hbm.xml"/>
    <mapping resource="Models/Hotel.hbm.xml"/>
    <mapping resource="Models/Clase.hbm.xml"/>
    <mapping resource="Models/Reservacion.hbm.xml"/>
    <mapping resource="Models/Habitacion.hbm.xml"/>
    <mapping resource="Models/Tipo.hbm.xml"/>
  </session-factory>
</hibernate-configuration>

hibernate.reveng.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-reverse-engineering PUBLIC "-//Hibernate/Hibernate Reverse Engineering DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-reverse-engineering-3.0.dtd">
<hibernate-reverse-engineering>
  <schema-selection match-catalog="Agencia"/>
  <table-filter match-name="Habitacion"/>
  <table-filter match-name="Clase"/>
  <table-filter match-name="Reservacion"/>
  <table-filter match-name="Avion"/>
  <table-filter match-name="Tipo"/>
  <table-filter match-name="Hotel"/>
  <table-filter match-name="Sucursal"/>
  <table-filter match-name="Vuelo"/>
  <table-filter match-name="Cliente"/>
</hibernate-reverse-engineering>

ClienteDAO.java

package DAO;

import Models.Cliente;
import java.util.List;
import org.hibernate.HibernateException;
import org.hibernate.Session;
import org.hibernate.Transaction;

/**
 *
 * @author GerardoAGL
 */
public class ClienteDAO {

    private Session sesion;
    private Transaction tx;

    public int guardar(Cliente cliente) {
        int id = 0;

        try{
            iniciaOperacion();
            id = (int) sesion.save(cliente);
            tx.commit();
        }catch(HibernateException e){
            manejaExcepcion(e);
        }finally{
            sesion.close();
        }

        return id;
    }

    public List<Cliente> buscar(String busqueda) {

        List<Cliente> clientes = null;
        String sql ="FROM Cliente WHERE "
                + "nombre LIKE '%"+busqueda+"%' OR "
                + "apellido LIKE '%"+busqueda+"%' OR "
                + "cedula LIKE '%"+busqueda+"%'";

        try{
            iniciaOperacion();
            clientes = sesion.createQuery(sql).list();
        }finally{
            sesion.close();
        }

        return clientes;
    }

    public void eliminar(Cliente cliente) {
        try{
            iniciaOperacion();
            sesion.delete(cliente);
            tx.commit();
        }catch(HibernateException e){
            manejaExcepcion(e);
        }finally{
            sesion.close();
        }
    }

    private void iniciaOperacion() throws HibernateException{

        sesion = NewHibernateUtil.getSessionFactory().openSession();
        tx = sesion.beginTransaction();
    }

    private void manejaExcepcion(HibernateException e) throws HibernateException {

        tx.rollback();
        throw new HibernateException("Ocurrió un error en la capa de acceso a datos", e);
    } 
}

This is the error I have

INFO: HHH000400: Using dialect: org.hibernate.dialect.SQLServer2012Dialect
sep 03, 2017 11:20:03 AM org.hibernate.engine.transaction.internal.TransactionFactoryInitiator initiateService
INFO: HHH000399: Using default transaction strategy (direct JDBC transactions)
sep 03, 2017 11:20:03 AM org.hibernate.hql.internal.ast.ASTQueryTranslatorFactory <init>
INFO: HHH000397: Using ASTQueryTranslatorFactory
Initial SessionFactory creation failed.org.hibernate.MappingException: Could not get constructor for org.hibernate.persister.entity.SingleTableEntityPersister
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at DAO.ClienteDAO.guardar(ClienteDAO.java:33)
    at Controllers.CreateClientController.guardarCliente(CreateClientController.java:67)
    at Controllers.CreateClientController.actionPerformed(CreateClientController.java:44)
    at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
    at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2348)
    at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
    at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
    at java.awt.Component.processMouseEvent(Component.java:6533)
    at javax.swing.JComponent.processMouseEvent(JComponent.java:3324)
    at java.awt.Component.processEvent(Component.java:6298)
    at java.awt.Container.processEvent(Container.java:2236)
    at java.awt.Component.dispatchEventImpl(Component.java:4889)
    at java.awt.Container.dispatchEventImpl(Container.java:2294)
    at java.awt.Component.dispatchEvent(Component.java:4711)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4888)
    at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4525)
    at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4466)
    at java.awt.Container.dispatchEventImpl(Container.java:2280)
    at java.awt.Window.dispatchEventImpl(Window.java:2746)
    at java.awt.Component.dispatchEvent(Component.java:4711)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:758)
    at java.awt.EventQueue.access$500(EventQueue.java:97)
    at java.awt.EventQueue$3.run(EventQueue.java:709)
    at java.awt.EventQueue$3.run(EventQueue.java:703)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:80)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:90)
    at java.awt.EventQueue$4.run(EventQueue.java:731)
    at java.awt.EventQueue$4.run(EventQueue.java:729)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:80)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:728)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)

The line that causes me the problem, of the ClienteDAO class is the following

Thanks in advance to anyone who can help me.