Saya cuba melakukan tindakan dalam skrin lain dari satu skrin tetapi setiap kali saya mengklik butang log masuk saya mendapat ralat ini -----> java.lang.NullPointerException: Tidak boleh memanggil "java.sql. Connection.prepareStatement(String)” kerana “this.conexao” kosong

Berikut ialah kod kelas

package br.com.mycompany.telas;

import java.sql.*;
import javax.swing.*;

public class TelaLogin extends javax.swing.JFrame {

    Connection conexao = null;
    PreparedStatement stm = null;
    ResultSet rs = null;

    public void logar() {
        String sql = "select * from usuarios where login=? and senha= ?";

        try {
            stm = conexao.prepareStatement(sql);
            stm.setString(1, txtUsuario.getText());
            stm.setString(1, txtSenha.getText());
            rs = stm.executeQuery();

            if (rs.next()) {
                TelaPrincipal principal = new TelaPrincipal();
                principal.setVisible(true);
            } else {
                JOptionPane.showMessageDialog(null, "deu ruim");
            }
        } catch (Exception e) {
            System.out.println(e);
        }
    }

    public TelaLogin() {
        initComponents();
    }

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

        jLabel1 = new javax.swing.JLabel();
        jLabel2 = new javax.swing.JLabel();
        txtUsuario = new javax.swing.JTextField();
        txtSenha = new javax.swing.JPasswordField();
        jButton1 = new javax.swing.JButton();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

        jLabel1.setText("Usuario");

        jLabel2.setText("Senha");

        jButton1.setText("Login");
        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()
                .addGap(146, 146, 146)
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING, false)
                    .addComponent(jLabel2, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                    .addComponent(jLabel1, javax.swing.GroupLayout.DEFAULT_SIZE, 58, Short.MAX_VALUE))
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING, false)
                    .addComponent(txtUsuario)
                    .addComponent(txtSenha, javax.swing.GroupLayout.DEFAULT_SIZE, 270, Short.MAX_VALUE))
                .addContainerGap(javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))
            .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
                .addContainerGap(283, Short.MAX_VALUE)
                .addComponent(jButton1)
                .addGap(258, 258, 258))
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addGap(87, 87, 87)
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                    .addComponent(jLabel1)
                    .addComponent(txtUsuario, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE))
                .addGap(58, 58, 58)
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                    .addComponent(jLabel2)
                    .addComponent(txtSenha, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE))
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, 53, Short.MAX_VALUE)
                .addComponent(jButton1)
                .addGap(49, 49, 49))
        );

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

    private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {                                         
        logar();
    }                                        

    public static void main(String args[]) {
        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                new TelaLogin().setVisible(true);
            }
        });
    }

    // Variables declaration - do not modify                     
    private javax.swing.JButton jButton1;
    private javax.swing.JLabel jLabel1;
    private javax.swing.JLabel jLabel2;
    private javax.swing.JPasswordField txtSenha;
    private javax.swing.JTextField txtUsuario;
    // End of variables declaration                   
}