Error filling JTextField with data from JTable [closed]

I have the problem that my program returns an error, which I don't know why it occurs, when collecting data from a JTableselected row and passing it to a series of JTextField, the error produced is this:

java.lang.ArrayIndexOutOfBoundsException: 0 >= 0

I attach code where I load the data in the textField

private void btModificarActionPerformed(java.awt.event.ActionEvent evt) {                                            

        int rowIndex = tabla.getSelectedRow();
        dni = String.valueOf(tabla.getValueAt(rowIndex,0));
        nombre = String.valueOf(tabla.getValueAt(rowIndex,1));
        ciudad = String.valueOf(tabla.getValueAt(rowIndex,2));


        tfDni.setText(dni);
        tfNombre.setText(nombre);
        tfCiudad.setText(ciudad);

        btInsertar.setEnabled(false);
        btBorrar.setEnabled(false);
        btListado.setEnabled(false);

    } 

Here I call the modification:

 private void btAceptarActionPerformed(java.awt.event.ActionEvent evt) {                                          
        modificar();  
    } 

Here if I make the modification:

public void modificar(){
       int rowIndex = tabla.getSelectedRow();

       try{      
       String dni = String.valueOf(tabla.getValueAt(rowIndex, 0));
       String nombre = String.valueOf(tabla.getValueAt(rowIndex, 1));
       String ciudad = String.valueOf(tabla.getValueAt(rowIndex, 2));
       String sql = "update clientes set nombre='"+nombre+"',ciudad='"+ciudad+"' where dni='"+dni+"'";
       /*PARA HACER QUE EL UPDATE COJA LOS VALORES DEL CAMPO DE TEXTO, HABRÍA QUE PASARLE EL tf.getText() que corresponda*/
       mysql.ejecutar(sql);
       }catch(Exception e){
           JOptionPane.showMessageDialog(null, "Error al Modificar");
       }
    }

The constructor of the application is like this, I think the fault may be there.

public class Ventana extends javax.swing.JFrame {
    MySQL mysql;
    Connection con;
    PreparedStatement pst;
    Statement st;
    DefaultTableModel modelo;
    String dni;
    String nombre;
    String ciudad;
    ResultSet rs;

public Ventana(MySQL mysql) throws SQLException {
    /*INSTANCIAMOS MYSQL*/
    this.mysql = new MySQL();
    initComponents();
    this.setVisible(true);
    con = mysql.MySQLConnection("pedidos", "root", "1234");
    modelo = new DefaultTableModel();
    tabla.setModel(modelo);

}

Here is the photo so you can see everything

Here I attach another photo that shows all the code

Finally I attach the method where I show what is in the database and get the metadata of the columns

when choosing the row it returns me that

It returns -1, that means, in my opinion, that it has not been updated, and indeed it is.

I'm sorry for answering delay