How to display the contents of a .txt file on the screen?

I am using JFrame to try to display the contents of txt files in a window and then delete the whole file. However, when I put it to display, it deletes the file but does not display on the screen that I created with JFrame . What could be happening?

Follow the code:

/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */
package Sistema;

import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import javax.swing.JOptionPane;

/**
 *
 * @author marqu
 */
public class NewJFrame extends javax.swing.JFrame {

    /**
     * Creates new form NewJFrame
     */
    public NewJFrame() {
        initComponents();
    }

    /**
     * This method is called from within the constructor to initialize the form.
     * WARNING: Do NOT modify this code. The content of this method is always
     * regenerated by the Form Editor.
     */
    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">                          
    private void initComponents() {

        lab = new javax.swing.JLabel();
        jButton1 = new javax.swing.JButton();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

        lab.setBackground(new java.awt.Color(204, 204, 255));
        lab.setFont(new java.awt.Font("Tahoma", 0, 14)); // NOI18N
        lab.setHorizontalAlignment(javax.swing.SwingConstants.CENTER);
        lab.setText("Controle da Catraca");

        jButton1.setText("Exibir historico");
        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()
                        .addComponent(lab, javax.swing.GroupLayout.PREFERRED_SIZE, 372, javax.swing.GroupLayout.PREFERRED_SIZE))
                    .addGroup(layout.createSequentialGroup()
                        .addGap(142, 142, 142)
                        .addComponent(jButton1)))
                .addContainerGap(18, Short.MAX_VALUE))
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addGap(38, 38, 38)
                .addComponent(jButton1)
                .addGap(18, 18, 18)
                .addComponent(lab, javax.swing.GroupLayout.DEFAULT_SIZE, 210, Short.MAX_VALUE)
                .addContainerGap())
        );

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

    private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {                                         
        // TODO add your handling code here:

        String dir = "C:/Users/marqu/OneDrive/Documentos/marco.txt"; //String com o diretorio do arquivo txt
        Path caminho = Paths.get(dir); //Informa o caminho do arquivo txt
        byte[] txt = null;

        try {
            while (true) {

                do {
                    txt = Files.readAllBytes(caminho);// Ler o arquivo txt
                } while (txt.length <= 0); //Enquanto o arquivo txt estiver vazio, fique preso nesse WHILE

                //System.out.println(); // METODO TEMPORARIO SO PARA VISUALIZAR
                lab.setText(new String(txt));
                Files.write(caminho, "".getBytes()); // Apaga tudo do arquivo txt
            }
        } catch (Exception e) {
            System.out.println(e.getMessage());
        }



    }                                        

    /**
     * @param args the command line arguments
     */
    public static void main(String args[]) {
        /* Set the Nimbus look and feel */
        //<editor-fold defaultstate="collapsed" desc=" Look and feel setting code (optional) ">
        /* If Nimbus (introduced in Java SE 6) is not available, stay with the default look and feel.
         * For details see http://download.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html 
         */
        try {
            for (javax.swing.UIManager.LookAndFeelInfo info : javax.swing.UIManager.getInstalledLookAndFeels()) {
                if ("Nimbus".equals(info.getName())) {
                    javax.swing.UIManager.setLookAndFeel(info.getClassName());
                    break;
                }
            }
        } catch (ClassNotFoundException ex) {
            java.util.logging.Logger.getLogger(NewJFrame.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (InstantiationException ex) {
            java.util.logging.Logger.getLogger(NewJFrame.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (IllegalAccessException ex) {
            java.util.logging.Logger.getLogger(NewJFrame.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (javax.swing.UnsupportedLookAndFeelException ex) {
            java.util.logging.Logger.getLogger(NewJFrame.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        }
        //</editor-fold>

        /* Create and display the form */
        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                new NewJFrame().setVisible(true);
            }
        });
    }

    // Variables declaration - do not modify                     
    private javax.swing.JButton jButton1;
    private javax.swing.JLabel lab;
    // End of variables declaration                   
}