尝试在空对象引用上调用虚拟方法

Question

 package com.example.ben.droidmice2;

 import android.hardware.Sensor;
 import android.hardware.SensorEvent;
 import android.hardware.SensorEventListener;
 import android.hardware.SensorManager;
 import android.support.v7.app.AppCompatActivity;
 import android.os.Bundle; 

 import com.example.ben.droidmice2.net.NetworkTask;

 public class MainActivity extends AppCompatActivity implements   SensorEventListener{

    private long lastUpdate = 0;
    private static final float TILT_THRESHOLD = 1.1f;

    private SensorManager sensorManager;

    //sensorManager = (SensorManager) getSystemService(SENSOR_SERVICE);

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }

    private void getAccelerometer(SensorEvent event) {
        Sensor mySensor = event.sensor;

        if (mySensor.getType() == Sensor.TYPE_ACCELEROMETER) {
            float x = event.values[0];
            float y = event.values[1];
            //float z = event.values[2];
            long curTime = System.currentTimeMillis();

            if ((curTime - lastUpdate) > (1000/60)) {
                long diffTime = (curTime - lastUpdate);
                lastUpdate = curTime;

                if(Math.abs(x) > TILT_THRESHOLD || Math.abs(y) > TILT_THRESHOLD) {
                    int dx = (int)(x  * diffTime);
                    int dy = (int)(y  * diffTime);

                    new NetworkTask().execute("MOVE " + dx + "," + dy);
                }
            }
        }
    }


    @Override
    public void onSensorChanged(SensorEvent sensorEvent) {
        if (sensorEvent.sensor.getType() == Sensor.TYPE_ACCELEROMETER) {
            getAccelerometer(sensorEvent);
        }
    }

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


    protected void onPause() {
        super.onPause();
        sensorManager.unregisterListener(this);
    }


    @Override
    public void onResume(){
        super.onResume();
        // register this class as a listener for the orientation and
        // accelerometer sensors
        sensorManager.registerListener(this,
                sensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER),
                SensorManager.SENSOR_DELAY_FASTEST);
    }

 }

我正在尝试制作一个可以移动手机并使手机像鼠标一样的应用程序 这是我的代码，告诉我我有一些问题 尝试在空对象引用上调用虚拟方法'android.hardware.Sensor android.hardware.SensorManager.getDefaultSensor（int）' 对我来说，这里的OnResume（）里面没有问题 有人帮忙吗？