TextArea and ScollPane

[Smilidon Sapiens]

Hi,
I wrote this code:

class TabbedPaneContainer implements ActionListener
{
  private JTabbedPane tabPane = new JTabbedPane();
  private JButton button = new JButton("Schlie�en");
  private JPanel tabPanel = new JPanel();
  private JTextArea textArea = new JTextArea(30,50);
  private JScrollPane scrollPane = new JScrollPane(textArea,JScrollPane.VERTICAL_SCROLLBAR_AS_NEEDED, JScrollPane.HORIZONTAL_SCROLLBAR_AS_NEEDED);
  private JPanel pm_panel;
  public TabbedPaneContainer(String str, JPanel mainPanel)
  {
    this.pm_panel = mainPanel;
    button.addActionListener(this);
    textArea.setEditable(true);
    textArea.setText("sdssssssssssssss");
    tabPanel.setLayout(new BorderLayout());
    textArea.setPreferredSize(new Dimension(100, 100));
    scrollPane.setMinimumSize(new Dimension(200, 200));
    scrollPane.setViewportView(textArea);
    scrollPane.add(textArea);
    tabPanel.add("North", scrollPane);
    tabPanel.add("South", button);
    tabPane.addTab(str, tabPanel);
    mainPanel.add(tabPane);
  }
  public void actionPerformed(ActionEvent e)
  {
    if (e.getSource() == button)
    {
      tabPane.getParent().remove(tabPane);
      pm_panel.repaint();
      button = null;
      scrollPane = null;
      tabPanel = null;
      tabPane = null;
      textArea = null;
    }
  }
}

The question:
If I create a new object of this class in my JFrame ("JFrame.add(panel); TabbedPaneContainer cont = new TabbedPaneContainer("Test", panel);"), the ScrollPane has always the size 0. How can draw the ScrollPane correctly?
thanks...