How To Make Countdown Timer

As a part of my app I would like to include a countdown timer.

I would much rather have the timer count in minutes and seconds, so any good tutorial to set such a timer up I would rather they point me in that direction first (I have looked)

My problem is that I'm getting this error when I press the button to open the activity

02-27 21:07:47.776: E/AndroidRuntime(711): Uncaught handler: thread main exiting due to uncaught exception
02-27 21:07:47.786: E/AndroidRuntime(711): java.lang.RuntimeException: Unable to start activity ComponentInfo{converter.units.kitchen/converter.units.kitchen.Timer}: java.lang.NullPointerException

I currently have the following java activity

import
android.app.Activity;
import
android.os.Bundle;
import
android.os.CountDownTimer;
import

[code]....

It was working fine when I had a fixed number as the starting value, although since reverting to a fixed number it still does not work.