Android development, onClickListener not working

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http://stackoverflow.com – I'm completely new to Android development and I'm working through a book (practical android 4 games development) to make a game called starfighter. This is a problem on the menu screen. The exit button used to work, and close the application down. Since I worked through playing music in the background, it no longer works. Clicking on exit stops the music, but doesn't close the application and I can't figure out why, any help please? The code is below: SFMainMenu.java: package com.example.starfighter; import android.app.Activity; import android.content.Intent; import android.os.Bundle; impo (HowTos)