NullPointerException when trying to use method from custom JPanel

I am trying to make a GUI for my game I created a while back, and I am running into a slight issue while running it.

I want to have the output print to a JTextField in an extended JPanel. However, when I run it, it comes up with this error:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at Classic.print(Classic.java:509)
    at Classic.play(Classic.java:43)
    at Karma.actionPerformed(Karma.java:134)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2346)
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:6525)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3324)
at java.awt.Component.processEvent(Component.java:6290)
at java.awt.Container.processEvent(Container.java:2234)
at java.awt.Component.dispatchEventImpl(Component.java:4881)
at java.awt.Container.dispatchEventImpl(Container.java:2292)
at java.awt.Component.dispatchEvent(Component.java:4703)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4898)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4533)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4462)
at java.awt.Container.dispatchEventImpl(Container.java:2278)
at java.awt.Window.dispatchEventImpl(Window.java:2750)
at java.awt.Component.dispatchEvent(Component.java:4703)
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$1.doIntersectionPrivilege(ProtectionDomain.java:75)
at java.security.ProtectionDomain$1.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$1.doIntersectionPrivilege(ProtectionDomain.java:75)
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)

Here is the applicable code:

(I will skip Karma.java:134 since this is just the play button.)

Classic.java

public class Classic extends Game {
    private static JFrame gui;
    private static GUIClassic newContentPane;
    ...
    public void play() {
        invokeLater(Classic::startGUI);
        //The next line is "Classic.java:43
        //length, difficulty, and log are all strings that were initialized when the game was instantiated
        print("Selected Options:\nLength: " + length + "\nDifficulty: " + difficulty + "\nOutput Log? " + log + "\n");
        ...
    }
    private static void startGUI() {
        gui = new JFrame("Karma :: Classic Mode");
        gui.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        newContentPane = new GUIClassic();
        newContentPane.setOpaque(true);
        gui.setContentPane(newContentPane);
        gui.pack();
        gui.setVisible(true);
    }
    private static void print(String text) {
        newContentPane.appendOutput(text);
    }

GUIClassic.java (The class used for the content pane)

public class GUIClassic extends JPanel implements ActionListener {
    private JTextArea output;
    ...
    public void appendOutput(String addition) {
        output.append(addition + "\n");
    }

Interestingly enough, the GUI does pop up and stay up after the exception occurs. It just doesn't print the output to the JTextArea and gets stuck.

If you need more code for context, let me know please and I will add it.

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