What is a Java NullPointerException? [duplicate]

Answer 1

Yes you are adding panel to the frame before creating object of JPanel. Anyway change your constructor with this:

public Executer() {
        JFrame frame = new JFrame("Execute Script");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(900, 400);

        MyPanel();
        Text();
        Buttons();
        Fields();

        frame.add(panel);
        frame.setVisible(true);

    }

thanks.