Can't register more than one Sensor in Android

I have a service which is called from an Activity. This Service Registers all Sensors and Registers Listener against them.

EDIT: I have removed the two lines in onSensorChange()

**sensorManager.unregisterListener(this);
        sensorManager = null;**

AND now it works. WHY?

The Problem is that when I do them one at a time, my application works fine but if I register more than one sensor, My application crashes.

Here is the OnCreate Function of my Service:

@Override
    public void onCreate() {
        super.onCreate();
        mContext = getApplicationContext();
        Toast.makeText(getApplicationContext(),"Service Created",Toast.LENGTH_SHORT).show();
        sensorManager = (SensorManager) getSystemService(SENSOR_SERVICE);

        //mAccelerometer = sensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER);
        mMagnetometer = sensorManager.getDefaultSensor(Sensor.TYPE_MAGNETIC_FIELD);
        //mGyro = sensorManager.getDefaultSensor(Sensor.TYPE_GYROSCOPE);
        //      mLinearAccelertion = sensorManager.getDefaultSensor(Sensor.TYPE_LINEAR_ACCELERATION);

        //sensorManager.registerListener(this, mAccelerometer, SensorManager.SENSOR_DELAY_NORMAL);
        sensorManager.registerListener(this, mMagnetometer, SensorManager.SENSOR_DELAY_NORMAL);
        //sensorManager.registerListener(this, mGyro, SensorManager.SENSOR_DELAY_NORMAL);
        //sensorManager.registerListener(this, mLinearAccelertion, SensorManager.SENSOR_DELAY_NORMAL);


    }

Here is my onSensorChanged Function:

    @Override
    public void onSensorChanged(SensorEvent event) {
        //Toast.makeText(mContext,"Heelooo" , Toast.LENGTH_SHORT).show();
        float[] values = event.values;
        Sensor mSensor = event.sensor;
        boolean res = CalculatePosition.calculateNewPosition(4.5F,5.3F);
        if(res){
            Toast.makeText(mContext, "Check", Toast.LENGTH_SHORT).show();
        }
        //CalculatePosition.test(event);
        sensorManager.unregisterListener(this);
        sensorManager = null;
}

I am starting the service like this in MyActivity onCreate Function:

intent = new Intent(this, CustomSensorService.class);
        startService(intent);

Your OnSensorChanged event is setting the sensorManager to Null so as soon as a sensor is fired no more can be set.

EDIT

If you want each sensor to use a different listener, use something like this instead of implementing the methods on your activity:

private void SetupSensors() {
    sensorManager.registerListener(MagneticListener, sensorManager.getDefaultSensor(Sensor.TYPE_MAGNETIC_FIELD), SensorManager.SENSOR_DELAY_NORMAL);
}

private SensorEventListener MagneticListener = new SensorEventListener() {

    @Override
    public void onSensorChanged(SensorEvent sensorEvent) {
        // Code here
    }

    @Override
    public void onAccuracyChanged(Sensor sensor, int i) {

    }
};