End session after 5 min when APP INACTIVE or pressed home button - android

General information for the app: I want this is script for secure data from other
Kind of development: Customization of existing app
Description of requirements/features: I USED AS BELOW
public class LogoutService extends Service {
public static CountDownTimer timer;

@Override
public void onCreate() {
// TODO Auto-generated method stub
super.onCreate();
timer = new CountDownTimer(1, 1000) {//1 * 60 * 1000, 1000
public void onTick(long millisUntilFinished) {
//Some code
// Log.v(Constants.TAG, "Service Started");
}

public void onFinish() {
// Log.v(SyncStateContract.Constants.TAG, "Call Logout by Service");
// Code for Logout
dialogBox();
Log.i("fr","fr");

stopSelf();
}
};
}
public void dialogBox() {
AlertDialog.Builder alertDialogBuilder = new AlertDialog.Builder(this);
alertDialogBuilder.setMessage("Your session has expired.");
alertDialogBuilder.setPositiveButton("Ok",
new DialogInterface.OnClickListener() {



@Override
public void onClick(DialogInterface arg0, int arg1) {
}
});

// alertDialogBuilder.setNegativeButton("cancel",
// new DialogInterface.OnClickListener() {
//

// @Override
// public void onClick(DialogInterface arg0, int arg1) {
//
// }
// });

AlertDialog alertDialog = alertDialogBuilder.create();
alertDialog.show();
}

@Override
public IBinder onBind(Intent intent) {
// TODO Auto-generated method stub
return null;



}
}

how to code end session after(logout) 5 min when android APP INACTIVE or pressed home button? i have used below code...... that for keep data on save.....................................................................

public class LogoutService extends Service {
public static CountDownTimer timer;

@Override
public void onCreate() {
// TODO Auto-generated method stub
super.onCreate();
timer = new CountDownTimer(1, 1000) {//1 * 60 * 1000, 1000
public void onTick(long millisUntilFinished) {
//Some code
// Log.v(Constants.TAG, "Service Started");
}

public void onFinish() {
// Log.v(SyncStateContract.Constants.TAG, "Call Logout by Service");
// Code for Logout
dialogBox();
Log.i("fr","fr");

stopSelf();
}
};
}
public void dialogBox() {
AlertDialog.Builder alertDialogBuilder = new AlertDialog.Builder(this);
alertDialogBuilder.setMessage("Your session has expired.");
alertDialogBuilder.setPositiveButton("Ok",
new DialogInterface.OnClickListener() {



@Override
public void onClick(DialogInterface arg0, int arg1) {
}
});

// alertDialogBuilder.setNegativeButton("cancel",
// new DialogInterface.OnClickListener() {
//

// @Override
// public void onClick(DialogInterface arg0, int arg1) {
//
// }
// });

AlertDialog alertDialog = alertDialogBuilder.create();
alertDialog.show();
}

@Override
public IBinder onBind(Intent intent) {
// TODO Auto-generated method stub
return null;



}
}
on MainActivity:

@Override
protected void onResume() {
// TODO Auto-generated method stub
super.onResume();

LogoutService.timer.start();
}

@Override
protected void onStop() {
// TODO Auto-generated method stub
super.onStop();
LogoutService.timer.cancel();
}
the error:

3-28 01:52:12.381 1250-1250/? E/AndroidRuntime: FATAL EXCEPTION: main
Process: com. . ., PID: 1250
java.lang.NullPointerException
Extra notes: