Java ME, why does this code throw an exception?

Java ME

The code below compiles but it crashes during execution. Look for "<---
crash here" in the code, that tells where the crash occur. The error is a
NullPointerException. The problem is that I don't know what causing the
exception. When I don't know what's causing the error, it is difficult to
fix it.

Any suggestions what may cause this exception?

public class HelloWorld extends MIDlet implements CommandListener {
private String[] task;

public void commandAction(Command c, Displayable d) {
     if (c == okCommand) {
           task[0] = "xxxxxx"; <--- crash here
           task[1] = "ddddddddd";
           task[2] = "eeeeeeeeee";
           try {
                 Image iconGreen = Image.createImage("/icon-green.png");
                 Image iconRed = Image.createImage("/icon-red.png");
                 imageArray[0] = iconGreen;
                 imageArray[1] = iconRed;
                 imageArray[2] = iconGreen;
            } catch (java.io.IOException err) {
            }
            taskList = new List("My Tasks", Choice.IMPLICIT, task,
imageArray);
            taskList.addCommand(exitCommand);
            taskList.addCommand(okCommand);
            taskList.setCommandListener(this);
           display.setCurrent(taskList);
}