Erro ao exibir em um jtable, porque?

package Visualizar;

import utilitarios.conexao;
import javax.swing.JOptionPane;
import java.sql.*;
import javax.swing.table.DefaultTableModel;

public class Visualizar_Cidades_Estados extends javax.swing.JFrame {

    int navega = 0;
    conexao mostrar;
    public Visualizar_Cidades_Estados() {
        initComponents();

        mostrar = new conexao();
        mostrar.conecta();
        mostrar.executeSQL("select * from cad_cidades");       ---------->[b]Aqui é uma tabela do banco cadastro[/b]
        mostrar.executeSQL("select * from cad_estados");       ---------->[b]Aqui é outra tabela do banco[/b]
        preencher_jtable();   ------------------------------------------------->[b]Aqui é o metodo de preencher o Jtable[/b]
    }

    
    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">                          
    private void initComponents() {

        painel_topo = new javax.swing.JPanel();
        label_topo = new javax.swing.JLabel();
        jScrollPane1 = new javax.swing.JScrollPane();
        jTable = new javax.swing.JTable();
        jButton1 = new javax.swing.JButton();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

        painel_topo.setBackground(new java.awt.Color(153, 153, 153));

        label_topo.setFont(new java.awt.Font("Tahoma", 1, 18)); // NOI18N
        label_topo.setText("Visualizar Cidades e Estados Cadastrados");

        javax.swing.GroupLayout painel_topoLayout = new javax.swing.GroupLayout(painel_topo);
        painel_topo.setLayout(painel_topoLayout);
        painel_topoLayout.setHorizontalGroup(
            painel_topoLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(painel_topoLayout.createSequentialGroup()
                .addGap(50, 50, 50)
                .addComponent(label_topo)
                .addContainerGap(64, Short.MAX_VALUE))
        );
        painel_topoLayout.setVerticalGroup(
            painel_topoLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(painel_topoLayout.createSequentialGroup()
                .addContainerGap()
                .addComponent(label_topo, javax.swing.GroupLayout.PREFERRED_SIZE, 53, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addContainerGap(javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))
        );

        jTable.setModel(new javax.swing.table.DefaultTableModel(
            new Object [][] {
                {null, null},
                {null, null},
                {null, null},
                {null, null}
            },
            new String [] {
                "Cidades", "Estados"
            }
        ));
        jScrollPane1.setViewportView(jTable);

        jButton1.setText("Fechar");
        jButton1.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                jButton1ActionPerformed(evt);
            }
        });

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                    .addGroup(layout.createSequentialGroup()
                        .addContainerGap()
                        .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.TRAILING)
                            .addComponent(jScrollPane1, javax.swing.GroupLayout.Alignment.LEADING, javax.swing.GroupLayout.DEFAULT_SIZE, 482, Short.MAX_VALUE)
                            .addComponent(painel_topo, javax.swing.GroupLayout.Alignment.LEADING, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)))
                    .addGroup(layout.createSequentialGroup()
                        .addGap(218, 218, 218)
                        .addComponent(jButton1)))
                .addContainerGap())
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addContainerGap()
                .addComponent(painel_topo, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addGap(18, 18, 18)
                .addComponent(jScrollPane1, javax.swing.GroupLayout.PREFERRED_SIZE, 87, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED)
                .addComponent(jButton1)
                .addContainerGap(javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))
        );

        pack();
    }// </editor-fold>                        

    private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {                                         
    int fechar = JOptionPane.showConfirmDialog(null,"Deseja fechar? ","Visualização de dados",JOptionPane.YES_NO_OPTION);
    if (fechar == JOptionPane.YES_OPTION)
    {
        dispose();
    }

    }                                        

    /**
    * @param args the command line arguments
    */
    public static void main(String args[]) {
        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                new Visualizar_Cidades_Estados().setVisible(true);
            }
        });
    }

    // Variables declaration - do not modify                     
    private javax.swing.JButton jButton1;
    private javax.swing.JScrollPane jScrollPane1;
    private javax.swing.JTable jTable;
    private javax.swing.JLabel label_topo;
    private javax.swing.JPanel painel_topo;
    // End of variables declaration                   

    public void preencher_jtable()
    {
        jTable.getColumnModel().getColumn(0).setPreferredWidth(20);
        jTable.getColumnModel().getColumn(1).setPreferredWidth(20);
        
        DefaultTableModel modelo = (DefaultTableModel)jTable.getModel();
        //criamos um objeto modelo do tipo DefaultTableModel
        modelo.setNumRows(0);
        //toda vez que chamar este metodo, zera tudo e preenche de novo
        try
        {
            while (mostrar.resultset.next())
            modelo.addRow(new Object[]{mostrar.resultset.getString("cid_nome")
            ,mostrar.resultset.getString("est_nome")});
            mostrar.resultset.first();
        }
        catch(SQLException erro)
        {
            JOptionPane.showMessageDialog(null,"Erro ao listar");
        }
    }

}