Java Swing GUI

package gui; import javax.swing.*; import java.awt.event.ActionEvent; import java.awt.event.ActionListener; public class StartFrame { private JFrame frame; private JButton signUpButton; private JButton loginButton; private JPanel panel; public StartFrame () { loginButton.addActionListener(new ActionListener() { @Override public void actionPerformed(ActionEvent e) { new LoginFrame(); frame.dispose(); } }); signUpButton.addActionListener(new ActionListener() { @Override public void actionPerformed(ActionEvent e) { new SignUpFrame(); frame.dispose(); } }); // Setup frame // JFrame setup setupFrame(); } // setup frame private void setupFrame() { frame = new JFrame("Mini Social Media Application"); frame.setContentPane(panel); frame.setVisible(true); frame.setSize(500, 500); frame.setLocationRelativeTo(null); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); } } 

public static void main(String[] args) { new StartFrame(); System.out.println("Ended."); }

I'm getting an error in setupFrame method

at gui.StartFrame.setupFrame(StartFrame.java:37) at gui.StartFrame.(StartFrame.java:33)