NullPointerException na compilação

/*
 * TelaCarros.java
 *
 * Created on 18 de Outubro de 2007, 21:28
 */

package visao;


import javax.swing.JOptionPane;
import javax.swing.JPanel;
import java.util.HashMap;
import modelo.Carro;


/**
 *
 * @author  Nuno
 */
public class TelaCarros extends javax.swing.JFrame {
    HashMap<String, Carro> carros;
    /** Creates new form TelaCarros */
    public TelaCarros() {
        initComponents();
         HashMap<String,Carro> carros = new HashMap<String,Carro>();
    }
    
    /** 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.
     */
    // <editor-fold defaultstate="collapsed" desc=" Código Gerado ">                          
    private void initComponents() {
        jLabel1 = new javax.swing.JLabel();
        jSeparator1 = new javax.swing.JSeparator();
        txtPlaca = new javax.swing.JTextField();
        txtAno = new javax.swing.JTextField();
        txtModelo = new javax.swing.JTextField();
        txtCor = new javax.swing.JTextField();
        jLabel2 = new javax.swing.JLabel();
        jLabel3 = new javax.swing.JLabel();
        jLabel4 = new javax.swing.JLabel();
        jLabel5 = new javax.swing.JLabel();
        jButton1 = new javax.swing.JButton();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
        jLabel1.setFont(new java.awt.Font("Tahoma", 1, 12));
        jLabel1.setText("Aluguel De Carros");

        jSeparator1.setOrientation(javax.swing.SwingConstants.VERTICAL);

        jLabel2.setFont(new java.awt.Font("Tahoma", 1, 11));
        jLabel2.setText("Placa");

        jLabel3.setFont(new java.awt.Font("Tahoma", 1, 11));
        jLabel3.setText("Ano");

        jLabel4.setFont(new java.awt.Font("Tahoma", 1, 11));
        jLabel4.setText("Modelo");

        jLabel5.setFont(new java.awt.Font("Tahoma", 1, 11));
        jLabel5.setText("Cor");

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

        org.jdesktop.layout.GroupLayout layout = new org.jdesktop.layout.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(org.jdesktop.layout.GroupLayout.LEADING)
            .add(layout.createSequentialGroup()
                .add(layout.createParallelGroup(org.jdesktop.layout.GroupLayout.LEADING)
                    .add(layout.createSequentialGroup()
                        .add(layout.createParallelGroup(org.jdesktop.layout.GroupLayout.LEADING)
                            .add(layout.createSequentialGroup()
                                .add(42, 42, 42)
                                .add(layout.createParallelGroup(org.jdesktop.layout.GroupLayout.TRAILING, false)
                                    .add(layout.createSequentialGroup()
                                        .add(txtPlaca, org.jdesktop.layout.GroupLayout.DEFAULT_SIZE, 77, Short.MAX_VALUE)
                                        .addPreferredGap(org.jdesktop.layout.LayoutStyle.RELATED))
                                    .add(txtAno)
                                    .add(org.jdesktop.layout.GroupLayout.LEADING, txtModelo)
                                    .add(org.jdesktop.layout.GroupLayout.LEADING, txtCor)
                                    .add(org.jdesktop.layout.GroupLayout.LEADING, jButton1)))
                            .add(layout.createSequentialGroup()
                                .add(61, 61, 61)
                                .add(jLabel4))
                            .add(layout.createSequentialGroup()
                                .add(69, 69, 69)
                                .add(jLabel5))
                            .add(layout.createSequentialGroup()
                                .add(66, 66, 66)
                                .add(jLabel2))
                            .add(layout.createSequentialGroup()
                                .add(71, 71, 71)
                                .add(jLabel3)))
                        .add(93, 93, 93)
                        .add(jSeparator1, org.jdesktop.layout.GroupLayout.PREFERRED_SIZE, 18, org.jdesktop.layout.GroupLayout.PREFERRED_SIZE))
                    .add(layout.createSequentialGroup()
                        .add(150, 150, 150)
                        .add(jLabel1)))
                .addContainerGap(171, Short.MAX_VALUE))
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(org.jdesktop.layout.GroupLayout.LEADING)
            .add(layout.createSequentialGroup()
                .add(jLabel1)
                .add(7, 7, 7)
                .add(jSeparator1, org.jdesktop.layout.GroupLayout.DEFAULT_SIZE, 278, Short.MAX_VALUE))
            .add(layout.createSequentialGroup()
                .add(41, 41, 41)
                .add(jLabel2)
                .addPreferredGap(org.jdesktop.layout.LayoutStyle.RELATED)
                .add(txtPlaca, org.jdesktop.layout.GroupLayout.PREFERRED_SIZE, org.jdesktop.layout.GroupLayout.DEFAULT_SIZE, org.jdesktop.layout.GroupLayout.PREFERRED_SIZE)
                .add(9, 9, 9)
                .add(jLabel3)
                .addPreferredGap(org.jdesktop.layout.LayoutStyle.RELATED)
                .add(txtAno, org.jdesktop.layout.GroupLayout.PREFERRED_SIZE, org.jdesktop.layout.GroupLayout.DEFAULT_SIZE, org.jdesktop.layout.GroupLayout.PREFERRED_SIZE)
                .addPreferredGap(org.jdesktop.layout.LayoutStyle.RELATED)
                .add(jLabel4)
                .addPreferredGap(org.jdesktop.layout.LayoutStyle.RELATED)
                .add(txtModelo, org.jdesktop.layout.GroupLayout.PREFERRED_SIZE, org.jdesktop.layout.GroupLayout.DEFAULT_SIZE, org.jdesktop.layout.GroupLayout.PREFERRED_SIZE)
                .addPreferredGap(org.jdesktop.layout.LayoutStyle.RELATED)
                .add(jLabel5)
                .addPreferredGap(org.jdesktop.layout.LayoutStyle.RELATED)
                .add(txtCor, org.jdesktop.layout.GroupLayout.PREFERRED_SIZE, org.jdesktop.layout.GroupLayout.DEFAULT_SIZE, org.jdesktop.layout.GroupLayout.PREFERRED_SIZE)
                .add(21, 21, 21)
                .add(jButton1)
                .addContainerGap(34, Short.MAX_VALUE))
        );
        pack();
    }// </editor-fold>                        

    private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {                                         
    // não deixa entrar com os campos em branco
        if(txtPlaca.getText().equals("")){
            JOptionPane.showMessageDialog(this, "Informe a placa");
            txtPlaca.requestFocus();
            return;
        }
        
        // verifica se é uma placa válida
        String placa = txtPlaca.getText().toUpperCase();
        if(!placa.matches("[a-zA-z]{3}[0-9]{4}")){
            JOptionPane.showMessageDialog(this, "Use formato AAA9999");
            txtPlaca.requestFocus();
            return;
        }
        
        // verifica se já existe
        if(carros.containsKey(placa)){
            JOptionPane.showMessageDialog(this, "Placa já cadastrada.");
            txtPlaca.requestFocus();
            return;
        }
        
        // verifica campos em branco
        if(txtAno.getText().equals("")){
            JOptionPane.showMessageDialog(this, "Informe o ano");
            txtAno.requestFocus();
            return;
        }
        
        
        // tenta converter o ano em inteiro
        int ano;
        try {
            ano = Integer.parseInt(txtAno.getText());
        } catch (NumberFormatException ex) {
            JOptionPane.showMessageDialog(this, "Informe o ano");
            txtAno.requestFocus();
            return;
        }
          catch (NullPointerException ex)   {
           JOptionPane.showMessageDialog(this, "Informe o ano");
            txtAno.requestFocus();
            return;
        }
         if(txtCor.getText().equals("")){
            JOptionPane.showMessageDialog(this, "Informe a cor");
            txtCor.requestFocus();
            return;
        }
        
        // cria um carro novo
        String modelo = txtModelo.getText();
        String cor = txtCor.getText();
        Carro car = new Carro(placa, modelo, ano, cor);
        //guarda o carro no HASHMAP
        carros.put(placa, car);
        
        // avisa e limpa
        JOptionPane.showMessageDialog(this, "Carro cadastrado");
      
        txtPlaca.requestFocus();     
    }                                        
    
    /**
     * @param args the command line arguments
     */
    public static void main(String args[]) {
        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                new TelaCarros().setVisible(true);
            }
        });
    }
    
    // Declaração de variáveis - não modifique                     
    private javax.swing.JButton jButton1;
    private javax.swing.JLabel jLabel1;
    private javax.swing.JLabel jLabel2;
    private javax.swing.JLabel jLabel3;
    private javax.swing.JLabel jLabel4;
    private javax.swing.JLabel jLabel5;
    private javax.swing.JSeparator jSeparator1;
    private javax.swing.JTextField txtAno;
    private javax.swing.JTextField txtCor;
    private javax.swing.JTextField txtModelo;
    private javax.swing.JTextField txtPlaca;
    // Fim da declaração de variáveis                   
    
}