android - 加速度計データ

Question

このリンクから加速度計データを記録するアプリを構築しようとしています: http://mobile.tutsplus.com/tutorials/android/android-barometer-logger-acquiring-sensor-data/

問題は、エラーが発生していて、なぜこれらのエラーが発生するのか理解できないということです。以下にコードを貼り付けます。

import java.text.SimpleDateFormat;
import java.util.Date;



import android.hardware.Sensor;
import android.hardware.SensorEvent;
import android.hardware.SensorEventListener;
import android.hardware.SensorListener;
import android.hardware.SensorManager;
import android.os.Bundle;
import android.app.Activity;
import android.content.Intent;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;
import android.support.v4.app.NavUtils;

public class MainActivity extends Activity implements SensorEventListener {

    SensorManager sensorManager = null;
    Sensor sensor = sensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER);
    SensorEventListener myListenerInstance = null;

    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        sensorManager = (SensorManager) getSystemService(SENSOR_SERVICE);
        setContentView(R.layout.activity_main);
    }
    @Override
    public void onAccuracyChanged(Sensor sensor, int accuracy) {
        // TODO Auto-generated method stub

    }
    @Override
    public void onSensorChanged(SensorEvent event) {
        // TODO Auto-generated method stub
        sensorManager.registerListener(myListenerInstance, sensor,
                SensorManager.SENSOR_DELAY_NORMAL);
        long timestamp = event.timestamp;
        float value = event.values[0];

        // String res=String.valueOf(event.values[0])+"#"+String.valueOf(event.values[1])+"#"+String.valueOf(event.values[2]);

        // Log.d("test", res);
    }

    protected void onResume(){
        super.onResume();
        sensorManager.registerListener(myListenerInstance, sensor,
                SensorManager.SENSOR_DELAY_NORMAL);
    }
    protected void onStop(){

        sensorManager.unregisterListener(myListenerInstance);
        super.onStop();
    }
}

私が得ているエラーはここにあります:

06-22 21:44:27.322: E/AndroidRuntime(9918): FATAL EXCEPTION: main
06-22 21:44:27.322: E/AndroidRuntime(9918): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.rasam.sampleproject/com.rasam.sampleproject.MainActivity}: java.lang.NullPointerException
06-22 21:44:27.322: E/AndroidRuntime(9918):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java)
06-22 21:44:27.322: E/AndroidRuntime(9918):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java)
06-22 21:44:27.322: E/AndroidRuntime(9918):     at android.app.ActivityThread.access$600(ActivityThread.java)
06-22 21:44:27.322: E/AndroidRuntime(9918):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java)
06-22 21:44:27.322: E/AndroidRuntime(9918):     at android.os.Handler.dispatchMessage(Handler.java)
06-22 21:44:27.322: E/AndroidRuntime(9918):     at android.os.Looper.loop(Looper.java)
06-22 21:44:27.322: E/AndroidRuntime(9918):     at android.app.ActivityThread.main(ActivityThread.java)
06-22 21:44:27.322: E/AndroidRuntime(9918):     at java.lang.reflect.Method.invokeNative(Native Method)
06-22 21:44:27.322: E/AndroidRuntime(9918):     at java.lang.reflect.Method.invoke(Method.java)
06-22 21:44:27.322: E/AndroidRuntime(9918):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java)
06-22 21:44:27.322: E/AndroidRuntime(9918):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java)
06-22 21:44:27.322: E/AndroidRuntime(9918):     at dalvik.system.NativeStart.main(Native Method)
06-22 21:44:27.322: E/AndroidRuntime(9918): Caused by: java.lang.NullPointerException
06-22 21:44:27.322: E/AndroidRuntime(9918):     at com.rasam.sampleproject.MainActivity.<init>(MainActivity.java:26)
06-22 21:44:27.322: E/AndroidRuntime(9918):     at java.lang.Class.newInstanceImpl(Native Method)
06-22 21:44:27.322: E/AndroidRuntime(9918):     at java.lang.Class.newInstance(Class.java)
06-22 21:44:27.322: E/AndroidRuntime(9918):     at android.app.Instrumentation.newActivity(Instrumentation.java)
06-22 21:44:27.322: E/AndroidRuntime(9918):     ... 12 more

なぜこれが起こっているのか誰か教えてもらえますか???