Why won't my Swing GUI switch from one GUI to the next?

Question

I'm building a simple chat server with a temporary username system. When the screen pops up, it starts with a simple screen asking for your username. You can put in anything you want, it's purely temporary (I'm also experimenting). The code tells the program to proceed, save the username into the code for temporary use, and enter the main chat server (which is just a GUI so far). But that's not what happens. Instead, Java quite rudely tells me that I've made a mistake and that I'm a bad person. I'm not sure what I'm doing wrong or how it is happening, as everything seems to be fine and I thought that my button listeners were correct. What is my error, and how can I fix it? Here's my code:

package coltGUI;

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Font;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTextArea;
import javax.swing.JTextField;
import javax.swing.UIManager;

public class MainGUI {

    String username;

    public static void main(String[] args) {
        try {
            UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
        } catch (Exception e) {
            e.printStackTrace();
        }

        MainGUI gui = new MainGUI();
        gui.preDisplay();
    }

    JFrame frame;
    JButton sendMessage;
    JTextField messageBox;
    JTextArea chatBox;
    JTextArea usernameChooser;
    JFrame preFrame;

    private void preDisplay() {
        JFrame preFrame = new JFrame("Choose your username!(Colt chat v 0.1");
        JTextField usernameChooser = new JTextField();
        JLabel chooseUsernameLabel = new JLabel("Pick a username:");
        JButton enterServer = new JButton("Enter Chat Server");
        JPanel prePanel = new JPanel(new GridBagLayout());
        GridBagConstraints preRight = new GridBagConstraints();
        preRight.anchor = GridBagConstraints.EAST;
        GridBagConstraints preLeft = new GridBagConstraints();
        preLeft.anchor = GridBagConstraints.WEST;
        preRight.weightx = 2.0;
        preRight.fill = GridBagConstraints.HORIZONTAL;
        preRight.gridwidth = GridBagConstraints.REMAINDER;
        preFrame.add(BorderLayout.CENTER, prePanel);
        prePanel.add(chooseUsernameLabel, preLeft);
        prePanel.add(usernameChooser, preRight);
        preFrame.add(BorderLayout.SOUTH, enterServer);
        preFrame.setVisible(true);
        preFrame.setSize(300, 300);

        enterServer.addActionListener(new enterServerButtonListener());
    }

    public void display() {

        frame = new JFrame("Colt Chat (Alpha 0.1)");
        JPanel southPanel = new JPanel();

        frame.add(BorderLayout.SOUTH, southPanel);
        southPanel.setBackground(Color.BLUE);
        southPanel.setLayout(new GridBagLayout());

        messageBox = new JTextField(30);
        sendMessage = new JButton("Send Message");
        chatBox = new JTextArea();
        chatBox.setEditable(false);
        frame.add(new JScrollPane(chatBox), BorderLayout.CENTER);

        chatBox.setLineWrap(true);

        GridBagConstraints left = new GridBagConstraints();
        left.anchor = GridBagConstraints.WEST;
        GridBagConstraints right = new GridBagConstraints();
        right.anchor = GridBagConstraints.EAST;
        right.weightx = 2.0;

        southPanel.add(messageBox, left);
        southPanel.add(sendMessage, right);

        chatBox.setFont(new Font("Serif", Font.PLAIN, 15));
        sendMessage.addActionListener(new sendMessageButtonListener());
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(470, 300);
    }

    class sendMessageButtonListener implements ActionListener {
        public void actionPerformed(ActionEvent event) {
            if (messageBox.getText().length() < 1) {
                // do nothing 
            } else if (messageBox.getText().equals(".clear")) {
                chatBox.setText("Cleared all messages\n");
                messageBox.setText("");
            } else {
                chatBox.append("<" + username + ">:" + messageBox.getText() + "\n");
                messageBox.setText("");
            }
        }
    }

    class enterServerButtonListener implements ActionListener {
        public void actionPerformed(ActionEvent event) {
            username = usernameChooser.getText();
            preFrame.setVisible(false);
            frame.setVisible(true);

        }

    }
}

And here's that error. It occurs when I try and press the 'Enter Server' button on the choose username GUI:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at coltGUI.MainGUI$enterServerButtonListener.actionPerformed(MainGUI.java:113)
    at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
    at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
    at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
    at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
    at java.awt.Component.processMouseEvent(Unknown Source)
    at javax.swing.JComponent.processMouseEvent(Unknown Source)
    at java.awt.Component.processEvent(Unknown Source)
    at java.awt.Container.processEvent(Unknown Source)
    at java.awt.Component.dispatchEventImpl(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
    at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
    at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Window.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
    at java.awt.EventQueue.access$000(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue$4.run(Unknown Source)
    at java.awt.EventQueue$4.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue.dispatchEvent(Unknown Source)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.run(Unknown Source)

Answer 1

Because the fields your listener is relying on haven't been initialized

JFrame frame;
JButton sendMessage;
JTextField messageBox;
JTextArea chatBox;
JTextArea usernameChooser;
JFrame preFrame;

private void preDisplay() {
    // All the above fields have been redeclared here and are local to the constrcutor
    JFrame preFrame = new JFrame("Choose your username!(Colt chat v 0.1");
    JTextField usernameChooser = new JTextField();
    JLabel chooseUsernameLabel = new JLabel("Pick a username:");
    JButton enterServer = new JButton("Enter Chat Server");
    JPanel prePanel = new JPanel(new GridBagLayout());
    GridBagConstraints preRight = new GridBagConstraints();
    preRight.anchor = GridBagConstraints.EAST;
    GridBagConstraints preLeft = new GridBagConstraints();

Should look more like...

JFrame frame;
JButton sendMessage;
JTextField messageBox;
JTextArea chatBox;
JTextArea usernameChooser;
JFrame preFrame;

private void preDisplay() {
    // All the above fields have been redeclared here and are local to the constrcutor
    preFrame = new JFrame("Choose your username!(Colt chat v 0.1");
    usernameChooser = new JTextField();
    JLabel chooseUsernameLabel = new JLabel("Pick a username:");
    JButton enterServer = new JButton("Enter Chat Server");
    JPanel prePanel = new JPanel(new GridBagLayout());