Error executing a prepared statement

I have an error when executing a method that returns the result of a query. The error he gives me is this:

  

'Warning: StandardWrapperValve [jsp]: Servlet.service () for   servlet jsp threw exception java.lang.NullPointerException at
  com.sun.common.util.logging.LoggingOutputStream $ LoggingPrintStream.println (Logg   ingOutputStream.java:228) at
  org.apache.felix.gogo.runtime.threadio.ThreadPrintStream.println (ThreadPrintStr   eam.java:205) at
  DAOSIMP.DaoPersonaImp.consultarAspirantesQuantity (DaoPersonaImp.java:833) '

I already checked the connection and it seems that everything is fine. Attached the connection:

                package Conexion;
                import java.sql.*;

                public class Conexion {  
                    protected Connection conexion;
                    protected final String URL_DB= "jdbc:mysql://localhost:80/nombre_base_de_datos";
                    protected final String JDBC_DRIVER ="com.mysql.jdbc.Driver";
                    protected final String CONSTRASENA_DB="contrasena";
                    protected final String USUARIO_DB="usuario_BD";

                    public Conexion() {
                    }
                    public Connection conexion(){
                        return this.conexion;
                    }
                    public void conectar(){

                        try{
                            conexion= DriverManager.getConnection(URL_DB, USUARIO_DB,CONSTRASENA_DB); 
                            Class.forName(JDBC_DRIVER);
                        }catch (SQLException e){

                            e.getMessage();
                        }catch(ClassNotFoundException e){
                            System.out.println("Mensaje de error de base de datos" + e.getMessage());

                        }
                    }

                    public void desconectar(){
                        try{
                            if(conexion!=null){
                                if(conexion.isClosed()){
                                    conexion.close();
                                }
                            }
                        }catch(SQLException e){
                            System.out.println(e.getMessage());
                        }
                    }

                }

I enclose the class that extends the connection and the method in which I execute the query:

                package DAOSIMP;

                import Clases.Persona;
                import Conexion.Conexion;
                import DAOS.DaoPersona;
                import java.sql.PreparedStatement;
                import java.sql.ResultSet;
                import java.sql.SQLException;
                import java.util.ArrayList;

                public class DaoPersonaImp extends Conexion implements DaoPersona{

                    @Override
                    public long consultarCantidadAspirantes() {
                        ResultSet rs;
                        long cantidad=0;
                        try {
                            this.conectar();
                            String sql="SELECT COUNT(*) FROM persona WHERE tipo_persona='A';";

                            PreparedStatement stm=this.conexion.prepareStatement(sql);//Esta es la linea 833 que dice el error
                            rs = stm.executeQuery();
                            while(rs.next()){
                                cantidad=rs.getLong(1);
                            }
                            return cantidad;
                        }catch(SQLException e){
                            System.out.println(e.getMessage());
                            return 0;
                        }finally{
                            this.desconectar();
                        }
                    }
                }

And I'm just calling the method like this:

                DaoPersona daoPersona=new DaoPersonaImp();
                long cantidadAspirantes=daoPersona.consultarCantidadAspirantes();