Java encryption using AES

I am trying to create an encryption system using the AES class:

package Source;
import java.security.MessageDigest;
import java.util.Arrays;
import javax.crypto.KeyGenerator;
import javax.crypto.SecretKey;
import javax.crypto.spec.SecretKeySpec;
import javax.crypto.spec.IvParameterSpec;

import javax.crypto.Cipher;
import javax.crypto.spec.IvParameterSpec;
import javax.crypto.spec.SecretKeySpec;

public class AES {
  static String IV = "AAAAAAAAAAAAAAAA";
  static String plaintext = "test text 123\0\0\0"; /*Note null padding*/
  static String encryptionKey = "H4tch4repratygonetowil5h4kers";
  public static void main(String [] args) {
    try {

      System.out.println("==Java==");
      System.out.println("plain:   " + plaintext);

      byte[] cipher = encrypt(plaintext, encryptionKey);

      System.out.print("cipher:  ");
      for (int i=0; i<cipher.length; i++)
        System.out.print(new Integer(cipher[i])+" ");
      System.out.println("");

      String decrypted = decrypt(cipher, encryptionKey);

      System.out.println("decrypt: " + decrypted);

    } catch (Exception e) {
      e.printStackTrace();
    } 
  }

  public static byte[] encrypt(String plainText, String encryptionKey) throws Exception {
    Cipher cipher = Cipher.getInstance("AES/CBC/NoPadding", "SunJCE");
    SecretKeySpec key = new SecretKeySpec(encryptionKey.getBytes("UTF-8"), "AES");
    cipher.init(Cipher.ENCRYPT_MODE, key,new IvParameterSpec(IV.getBytes("UTF-8")));
    return cipher.doFinal(plainText.getBytes("UTF-8"));
  }

  private static String decrypt(byte[] cipherText, String encryptionKey) throws Exception{
    Cipher cipher = Cipher.getInstance("AES/CBC/NoPadding", "SunJCE");
    SecretKeySpec key = new SecretKeySpec(encryptionKey.getBytes("UTF-8"), "AES");
    cipher.init(Cipher.DECRYPT_MODE, key,new IvParameterSpec(IV.getBytes("UTF-8")));
    return new String(cipher.doFinal(cipherText),"UTF-8");
  }
}

Here is my code implementation:

import java.awt.EventQueue;

import javax.swing.JFrame;
import javax.swing.JOptionPane;
import javax.swing.JButton;
import java.awt.BorderLayout;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
import javax.swing.JTextPane;


import Source.AES;
import javax.swing.JTextField;
import javax.swing.JPasswordField;
import javax.swing.JTextArea;

public class first {

    private JFrame frame;
    private JPasswordField passwordField;
    private JTextArea txtrEnterTextHere;

    /**
     * Launch the application.
     */
    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                try {
                    first window = new first();
                    window.frame.setVisible(true);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });
    }

    /**
     * Create the application.
     */
    public first() {
        initialize();
    }

    /**
     * Initialize the contents of the frame.
     */
    private void initialize() {
        frame = new JFrame();
        frame.setBounds(100, 100, 450, 300);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        JOptionPane.showMessageDialog(null, "Welcome to Encryption System! ");
        JButton btnNewButton = new JButton("Send Info");
        btnNewButton.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent arg0) {

                String text = txtrEnterTextHere.getText();
                String pass = passwordField.getText();
                String str = null;
                try {
                    str = new String(AES.encrypt(text, pass));
                } catch (Exception e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
                txtrEnterTextHere.setText(str);

                JOptionPane.showMessageDialog(null, "Your intel has been encrypted!");

            }
        });
        frame.getContentPane().add(btnNewButton, BorderLayout.NORTH);

        passwordField = new JPasswordField();
        frame.getContentPane().add(passwordField, BorderLayout.SOUTH);

        txtrEnterTextHere = new JTextArea();
        txtrEnterTextHere.setText("ENTER TEXT HERE AND PASSWORD BELLOW!");

        frame.getContentPane().add(txtrEnterTextHere, BorderLayout.CENTER);
    }

}

How can I make the variable "str" ​​actually have the value of the encrypted text? I get an error at runtime and I have an empty field... can I change the byte[] to a string?

Update error:

java.lang.IllegalArgumentException: javax.crypto.spec.SecretKeySpec at Source.AES.encrypt(AES.java:41). Null key at (SecretKeySpec.java:96) at first $2.actionPerformed (first.java:62) at javax.swing.DefaultButtonModel.setPressed (unknown source) at javax.swing.DefaultButtonModel.fireActionPerformed (unknown source) ) at javax.swing.AbstractButton$Handler.actionPerformed (unknown source) at javax.swing.DefaultButtonModel.setPressed (unknown source) at javax.swing.AbstractButton$Handler.actionPerformed (unknown source) at javax.swing.JComponent .processMouseEvent (unknown source) at javax.swing.JComponent.processMouseEvent (unknown source) at javax.swing.plaf.basic.BasicButtonListener.mouseReleased (unknown source) at java.awt.Component.processEvent (unknown source) , java.awt.Container.processEvent (unknown source) at java.awt.Container.java.awt.Component.dispatchEventImpl (unknown source). java.awt.LightweightDispatcher. at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source). retargetMouseEvent(Unknown Source) at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source) at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source) Unknown source at java.awt.Container.dispatchEventImpl(java.awt.Window.dispatchEventImpl Unknown source of java.awt.Component.dispatchEventImpl (unknown source at java.awt.EventQueue.dispatchEventImpl) at java.awt.EventQueue.access$500 (unknown source) at java.awt.EventQueue$3.run (unknown source) at java.awt.EventQueue$3.run(unknown source) at java.security.AccessController.doPrivileged(native java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(method) method). Java's ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(unknown source), java.awt.EventQueue$4.run(unknown source), java.security.AccessController.doPrivileged(native method), java.awt.EventQueue$4.run (unknown source). security.ProtectionDomain $JavaSecurityAccessImpl.doIntersectionPrivilege (unknown source) in java.awt.