mouseListener方法的NullPointerException

Question

我正在尝试编写一个代码，只需单击一个点，两行三角形，三个三角形，然后重新启动4，但我第一次点击时仍然会得到nullpointerexception。这是我的代码：

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Point;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;

import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class TriangleClicker extends JFrame {
    private final int width = 700;
    private final int height = 800;
    private JPanel panel;
    private MouseListener listener;
    private int p1x;
    private int p1y;
    private int p2x;
    private int p2y;
    private int p3x;
    private int p3y;
    private int count = 0;
    private JComponent tri;

public TriangleClicker() {
    this.setSize(width, height);
    this.add(createPanel());
    this.tri = new MousePressListener();
    this.add(this.tri);
    listener = new MousePressListener();
    this.addMouseListener(listener);
}

public JPanel createPanel() {
    panel = new JPanel();
    panel.setSize(width, height);
    return panel;
}

class MousePressListener extends JComponent implements MouseListener  {
    public Graphics g;
    public void drawLine1(Graphics g) {
        g.setColor(Color.BLACK);
        g.drawLine(p1x, p1y, p1x, p1y);
    }

    public void drawLine2(Graphics g) {
        g.drawLine(p1x, p1y, p2x, p2y);
    }

    public void drawLine3(Graphics g) {
        g.drawLine(p3x, p3y, p1x, p1y);
        g.drawLine(p3x, p3y, p2x, p2y);
    }
    @Override
    public void mouseClicked(MouseEvent e) {
    }
    @Override
    public void mouseEntered(MouseEvent e) {
    }
    @Override
    public void mouseExited(MouseEvent e) {
    }
    public void mousePressed(MouseEvent e) {
        if (count == 0) {
            System.out.println("1");
            p1x = e.getX();
            p1y = e.getY();
            drawLine1(g);
            count++;
        }
        else if (count == 1) {
            p2x = e.getX();
            p2y = e.getY();
            drawLine2(g);
            count++;
        }
        else if(count == 2) {
            p3x = e.getX();
            p3y = e.getY();
            drawLine3(g);
            count++;
        }
        else if(count == 4) {
            g.clearRect(0, 0, WIDTH, HEIGHT);
            p1x = e.getX();
            p1y = e.getY();
            p2x = 0;
            p2y = 0;
            p3x = 0;
            p3y = 0;
            drawLine1(g);
            count = 1;
        }
    }
    @Override
    public void mouseReleased(MouseEvent e) {
    }
}
}

我的错误在makeLine1（）方法中尝试将颜色设置为黑色，并在mousePressed（）方法中尝试调用makeLine1（）。

感谢。