Java Swing BoxLayout не может быть общей ошибкой

Question

Я получаю ошибку BoxLayout, которой нельзя поделиться со следующим кодом, но у меня есть макет карты, и на одной из дочерних панелей есть BoxLayout. Я следовал учебному пособию и должен был настроить все так, как мне было нужно, и внезапно я получил ошибку, и это происходит в строке frame.pack().

public class OutputGUI {

      private ArrayList<JLabel> labels;
      JFrame frame;
      EmptyBorder border1;
      JPanel panel;
      ArrayList<Integer> messageCounts; // each panel on the card reperesents each character
      // panel0 will be the messageCount for the 0th character, etc 
      CardLayout c1;
      JPanel mainPanel; // main panel to switch between character ouput screens
      JPanel generalPanel; // generalPanel will be for example, showing output that's not relevant
      // to a character's turn, for example if the file was bad, it'd be output to this panel

      int numCharacters; // numCharacters cooresponds to the number of panels we have
      // note that it starts from 0 for the first character for easy indexing
      // for ex: when we want to add a new panel, we pass in numCharacters which gets incremented
      // every time we make a new panel, and then this number will be the string representation
      // for the card layout 
      HashMap<String, JPanel> cPanels;




public OutputGUI() {
      frame = new JFrame("Output Window");
      c1 = new CardLayout();
      mainPanel = new JPanel();

      mainPanel.setLayout(c1); // put the card layout on the main panel

      numCharacters = 0;
      generalPanel = new JPanel(new BoxLayout(generalPanel, BoxLayout.Y_AXIS));
      cPanels = new HashMap<String, JPanel>(); // initialize container of character panels
      cPanels.put("0", generalPanel);
      mainPanel.add(generalPanel, "g"); // g is to denote the general panel
      // whenever we want to show it
     c1.show(mainPanel, "m"); // show the general panel on startup
     border1 = new EmptyBorder(5, 5, 5, 5); // invisible border that gives spacing around text fields

     frame.add(mainPanel);        
     frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
     frame.pack();
     frame.setVisible(true);
  }


public static void main(String[] args) {
    // TODO Auto-generated method stub

    GUI tempGui = new GUI();
    OutputGUI op = new OutputGUI();
}