BasicTabbedPaneUI.tabPane becomes null - what could cause that and how can I debug it?

For better readability, I've moved some of the code that generates my GUI in classes. Unfortunately I had to make a mistake somewhere because now I'm getting this error:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException at javax.swing.plaf.basic.BasicTabbedPaneUI$ScrollableTabSupport.updateView(BasicTabbedPaneUI.java:3351)

It happens in javax/swing/plaf/basic/BasicTabbedPaneUI.java on this line:

       private void updateView() {
ERROR HERE: int tabPlacement = tabPane.getTabPlacement();
            int tabCount = tabPane.getTabCount();

There's a method in this class that can set the tabPane to null, but I'm definitelly not calling it:

public void uninstallUI(JComponent c) {
    uninstallKeyboardActions();
    uninstallListeners();
    uninstallDefaults();
    uninstallComponents();
    c.setLayout(null);

    this.tabPane = null;
}

I create the tabbed GUI using my TabbedWindow class:

public final JTabbedPane container;
  public TabbedWindow() {
    container = new JTabbedPane();
    container.setTabLayoutPolicy(JTabbedPane.SCROLL_TAB_LAYOUT);
    //More code but nothing with container
  }

I then append it to the main JPane:

public void createTabs(Container pane) {
    pane.setLayout(new GridLayout(1, 1));
    System.out.println("Creating tabbed window.");
    TabbedWindow win = new TabbedWindow();
    //Inbetween, I append some JFrames to the win.container
    //Adding to main pane:
    pane.add(win.container);
    System.out.println("Done.");

My questions now are:

1. What could be causing this problem in the java library? Why doesn't it throw more helpful exception?
2. How can I debug this problem and find out what's wrong?

I can't be sure about the real reasons, but the uninstallUI has been invoked because I used container.setTabLayoutPolicy(JTabbedPane.SCROLL_TAB_LAYOUT); before appending other elements.

Because I frankly doesn't understand how the whole thing works I decided to go the safe way:

  public void close() {
    container.setTabLayoutPolicy(JTabbedPane.SCROLL_TAB_LAYOUT);
    closed = true;
  }

And after calling close, nothing can be added to the GUI any more.