java - Java lang null ポインター例外

Question

次の簡単なコードはエラーを表示しませんが、実行されません。致命的な例外: main java.lang.RunTimeException: Unable to start activity.... java.lang.NullPointerException.

package com.example.mapstestmednex;
import android.app.Activity;
import android.location.Location;
import android.location.LocationListener;
import android.location.LocationManager;
import android.os.Bundle;
import android.view.Menu;
import android.widget.TextView;

public class MainActivity extends Activity {

private LocationManager lm;
private LocationListener localListenD;
public TextView tvLatitude;
public TextView tvLongitude;

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

    lm = (LocationManager) getSystemService(LOCATION_SERVICE);
    Location loc = lm.getLastKnownLocation("gps");

    tvLatitude.setText(Double.toString(loc.getLatitude()));
    tvLongitude.setText(Double.toString(loc.getLatitude()));

    localListenD = new DipsListLocListener();
    lm.requestLocationUpdates("gps", 30000L, 10.0f, localListenD);
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}

class DipsListLocListener implements LocationListener {

    @Override
    public void onLocationChanged(Location location) {
        tvLatitude.setText(Double.toString(location.getLatitude()));
        tvLongitude.setText(Double.toString(location.getLatitude()));
    }

    @Override
    public void onProviderDisabled(String arg0) {
    }

    @Override
    public void onProviderEnabled(String arg0) {
    }

    @Override
    public void onStatusChanged(String arg0, int arg1, Bundle arg2) {
    }

 }
}

xml ファイル

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/LinearLayout1"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="vertical"
tools:context=".MainActivity" >

<TextView
    android:id="@+id/lblLatitude"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:text="Latitude:" />

<TextView
    android:id="@+id/tvLatitude"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content" />

<TextView
    android:id="@+id/lblLongitude"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:text="Longitude:" />

<TextView
    android:id="@+id/tvLongitude"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content" />

</LinearLayout>

どうしたの？ありがとう。