Why is my JTextArea not showing up but my JToolBar is [Java]?

Question

I'm making a text editor in Java and I want to have a JFrame with a JPanel in it, and then a JTextArea and a JToolBar inside of the JPanel. The frame shows up how I want it, and the toolbar does although it's empty right now, but the JTextArea doesn't even appear. I thought I might need to specify a size but even when I did that it didn't make it appear.

I have three classes, the Main class, the GUI class, and the Screen class.

Here's the code in my Main class

public class Main {
    // Attributes:
    private static Screen screen = new Screen();
    private static GUI ui = new GUI();

    // Behaviors:
    public static void main(String[] args) {
        // Creates the JFrame with its preset attributes
        screen.createFrame();
        ui.addGUI();
        Screen.frame.pack();
        Screen.frame.setVisible(true);
    }

Here's the code in my Screen class:

public class Screen {
    // Attributes / Instance Variables:
    private final int WIDTH = 854;
    private final int HEIGHT = 480;
    private final static String TITLE = "Lightweight Java Text Editor";
    public static JFrame frame = new JFrame(TITLE);
    private Dimension dimensions = new Dimension(WIDTH, HEIGHT);

    // Behaviors / Methods:

    /*
     * Generates the frame to have a certain size, allows it to be resized, centers
     * it on the screen, terminates Java when the window is closed, and makes it
     * visible.
     * 
     * The width and height are ALWAYS initially 854x480!
     */
    public void createFrame() {
        frame.setPreferredSize(dimensions);
        frame.setResizable(true);
        frame.setLocationRelativeTo(null);
        // frame.pack(); makes the three
        // lines above actually show up
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }

    // Getters and Setters for private variables that may be needed
    public Dimension getDimensions() {
        return dimensions;
    }

    public void setDimensions(Dimension dimensions) {
        this.dimensions = dimensions;
    }

    public int getWIDTH() {
        return WIDTH;
    }

    public int getHEIGHT() {
        return HEIGHT;
    }

    public String getTITLE() {
        return TITLE;
    }
}

And finally, here's the code in my GUI class

public class GUI {
    // Attributes / Instance Variables:
    // The container
    JPanel panel = new JPanel();
    // The area where the user types
    JTextArea textEditingField = new JTextArea(600,200);
    // The tool-bar on the left side
    JToolBar tools = new JToolBar();

    public void addGUI() {
        Screen.frame.add(panel);
        Screen.frame.add(textEditingField);
        Screen.frame.add(tools);
    }
}

What have I done wrong?