JList displaying

Question

I'm trying to display JList without DefaultListModel. I use my own method add to add elements to JList, but it gives a lot of errors, such as

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException at javax.swing.JList$4.getSize(Unknown Source)

How it can be fixed?

Creating JList

    import laba2.ACrafts;
    import laba2.List;
    import javax.swing.*;

        public class MyForm extends JFrame{
                 JFrame formShow;
            private JList<ACrafts> list;
            public List li;
            private JButton btnAdd;

            public MyForm() {
                li = new List();
                    JPanel panelShow = new JPanel();

                formShow = new JFrame("Aircrafts");
                formShow.setBounds(100, 100, 700, 550);
                formShow.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                formShow.add(panelShow);
                panelShow.setLayout(null);

                        list = new JList<ACrafts>();
                list.setLayoutOrientation(JList.VERTICAL);
                list.setBounds(20, 20, 640, 150);
                        ACrafts[] arr;
                        arr = li.toArray();
                        list.setListData(arr);
                        panelShow.add(list);

                public void setData(ACrafts a) {
                li.add(a);      
            }
        }

Method add in List.java

    class Element{
        Element left;
        ACrafts data;
        Element right;  

    }

    public class List implements ListImpl{
         private Element first;
         private Element last;
         private int size;

            public List() {
                first = null;
                last = null;
                size = 0;
            }

        public void add(ACrafts e){
            Element elem = new Element();
            elem.data = e;
            if (size == 0)
                first = last = elem;
            else
                last.right = elem;
                elem.left = last;
                last = elem;
            size++;
        }
    }

Main function (setting data)

public static void main(String[] args) {

        ACrafts[] aircraft = new ACrafts[5];

        aircraft[0] = new ACrafts("Boeing 767-300F","США",7130,54000,13100,850);
        aircraft[1] = new ACrafts("Airbus 310-300F","Франция",39000,9600,12200,850);
        aircraft[2] = new ACrafts("Boeing 757-200F","США",7100,39780,12500,935);
        aircraft[3] = new ACrafts("IL-86","Украина",3300,15000,11000,950);
        aircraft[4] = new ACrafts("Avro RJ-100","Великобритания",2255,9500,10600,890);

        MyForm form = new MyForm();

        form.setData(aircraft[0]);
        form.setData(aircraft[1]);
        form.setData(aircraft[2]);
        form.setData(aircraft[3]);
        form.setData(aircraft[4]);

        form.formShow.setVisible(true);

    }

ACrafts.java

public class ACrafts implements Comparable<ACrafts> {
    public String name;
    public String production;
    public int flightRange;
    public int weight;
    public int flightHeight;
    public int speed;

    public ACrafts(String n, String pr, int fR, int w, int fH, int s){
        this.name=n;
        this.production=pr;
        this.flightRange=fR;
        this.weight=w;
        this.flightHeight=fH;
        this.speed=s;
    }
}