How to make friends mSensorManager and MainActivity?

Question

Android

Hello.
In main_activity added 2 buttons for stop and start changes events

public class MainActivity extends AppCompatActivity implements SensorEventListener {

    private TextView textView;
    private Button button;

     private final SensorManager mSensorManager;
     private final Sensor mAccelerometer;

    public MainActivity() {
    mSensorManager = (SensorManager)getSystemService(SENSOR_SERVICE);
    mAccelerometer = mSensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER);
    }

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


//        нажатие выход из приложения
        button =(Button) findViewById(R.id.button_exit);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub
                finish();
                System.exit(0);
            }
        });

//        нажатие старт измерений
        button =(Button) findViewById(R.id.button_start);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
              new MainActivity();
              onResume();
            }
        });
//        нажатие стоп измерений
        button =(Button) findViewById(R.id.button_stop);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                onPause();
            }
        });



    }


    @Override
    public void onSensorChanged(SensorEvent sensorEvent) {

    }

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

    }

    @Override
    protected void onResume() {
        super.onResume();
        mSensorManager.registerListener(this, mAccelerometer, SensorManager.SENSOR_DELAY_NORMAL);
    }

    @Override
    protected void onPause() {
        super.onPause();
        mSensorManager.unregisterListener(this);
    }

}

BUT when running in the emulator, it is forcibly stopped
. What is the problem and how to solve it?