Acessar arquivos ftp

FTPClient ftp = new FTPClient();
  
    try {
        ftp.connect("files.000webhost.com");
        
        ftp.login("usuario", "senha");
        
    } catch (IOException ex) {
        Logger.getLogger(atualiza_software.class.getName()).log(Level.SEVERE, null, ex);
    }
        
  
    
        
   
        try {
            ftp.changeWorkingDirectory ("FTP");
        } catch (IOException ex) {
            Logger.getLogger(atualiza_software.class.getName()).log(Level.SEVERE, null, ex);
        }
        String[] arq = null;
       
        try {
            
            arq = ftp.listNames();
        } catch (IOException ex) {
            Logger.getLogger(atualiza_software.class.getName()).log(Level.SEVERE, null, ex);
        }

        System.out.println ("Listando arquivos: \n");

        for (String f : arq){

              System.out.println(f);        
        
        JOptionPane.showMessageDialog(null, "conectado");
        
       }

Exception in thread “AWT-EventQueue-0” java.lang.NullPointerException

at Jframe.atualiza_software.jButton1ActionPerformed(atualiza_software.java:143)

at Jframe.atualiza_software.access$000(atualiza_software.java:24)

at Jframe.atualiza_software$1.actionPerformed(atualiza_software.java:56)

at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)

at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2348)

at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)

at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)

at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)

at java.awt.Component.processMouseEvent(Component.java:6533)

at javax.swing.JComponent.processMouseEvent(JComponent.java:3324)

at java.awt.Component.processEvent(Component.java:6298)

at java.awt.Container.processEvent(Container.java:2236)

at java.awt.Component.dispatchEventImpl(Component.java:4889)

at java.awt.Container.dispatchEventImpl(Container.java:2294)

at java.awt.Component.dispatchEvent(Component.java:4711)

at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4888)

at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4525)

at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4466)

at java.awt.Container.dispatchEventImpl(Container.java:2280)

at java.awt.Window.dispatchEventImpl(Window.java:2746)

at java.awt.Component.dispatchEvent(Component.java:4711)

at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:758)

at java.awt.EventQueue.access$500(EventQueue.java:97)

at java.awt.EventQueue$3.run(EventQueue.java:709)

at java.awt.EventQueue$3.run(EventQueue.java:703)

at java.security.AccessController.doPrivileged(Native Method)

at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76)

at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:86)

at java.awt.EventQueue$4.run(EventQueue.java:731)

at java.awt.EventQueue$4.run(EventQueue.java:729)

at java.security.AccessController.doPrivileged(Native Method)

at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76)

at java.awt.EventQueue.dispatchEvent(EventQueue.java:728)

at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)

at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)

at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)

at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)

at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)

at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)