这个程序为什么不能运行，是小弟我设的引用异常么

import java.awt.*;import java.awt.event.*;public class TFmath{    public static void main(String[] args){        new MyFrame();        }    }    class MyFrame extends Frame{    public MyFrame(){        setLayout(new FlowLayout());        TextField num1 = new TextField(10);        TextField num2 = new TextField(10);        TextField num3 = new TextField(15);                Label lab = new Label("+");                Button bun = new Button("=");        bun.addActionListener(new minitor(this));        add(num1);        add(lab);                add(num2);                add(bun);        add(num3);                pack();        setVisible(true);                }    }        class Minitor implements ActionListener{    MyFrame mf = null;    Minitor(MyFrame mf){        this.mf = mf;        }            public void actionPerformed(ActionEvent e){        int n1 = Integer.parseInt(mf.num1.getText());        int n2 = Integer.parseInt(mf.num2.getText());        int n3 = (n1 + n2);        mf.num3.setText(String.valueOf(n3));                }    }