在地图上使用叠加层时发生NullPointerException

04-21 14:17:46.188: ERROR/AndroidRuntime(1632): java.lang.NullPointerException
04-21 14:17:46.188: ERROR/AndroidRuntime(1632):     at com.android.internal.app.AlertController$AlertParams.<init>(AlertController.java:753)
04-21 14:17:46.188: ERROR/AndroidRuntime(1632):     at android.app.AlertDialog$Builder.<init>(AlertDialog.java:273)
04-21 14:17:46.188: ERROR/AndroidRuntime(1632):     at achint.major.MapItemizedOverlays.onTap(MapItemizedOverlays.java:34)
04-21 14:17:46.188: ERROR/AndroidRuntime(1632):     at com.google.android.maps.ItemizedOverlay.onTap(ItemizedOverlay.java:453)
04-21 14:17:46.188: ERROR/AndroidRuntime(1632):     at com.google.android.maps.OverlayBundle.onTap(OverlayBundle.java:83)
04-21 14:17:46.188: ERROR/AndroidRuntime(1632):     at com.google.android.maps.MapView$1.onSingleTapUp(MapView.java:346)
04-21 14:17:46.188: ERROR/AndroidRuntime(1632):     at android.view.GestureDetector.onTouchEvent(GestureDetector.java:506)
04-21 14:17:46.188: ERROR/AndroidRuntime(1632):     at com.google.android.maps.MapView.onTouchEvent(MapView.java:628)
04-21 14:17:46.188: ERROR/AndroidRuntime(1632):     at android.view.View.dispatchTouchEvent(View.java:3709)
04-21 14:17:46.188: ERROR/AndroidRuntime(1632):     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:852)

public class MapItemizedOverlays extends ItemizedOverlay {

private ArrayList<OverlayItem> mOverlays= new ArrayList<OverlayItem>();
Context mContext;


public MapItemizedOverlays(Drawable defaultMarker) {
    super(boundCenterBottom(defaultMarker));
    // TODO Auto-generated constructor stub
}

public MapItemizedOverlays(Drawable defaultmarker, Context context){
    super(defaultmarker);
     mContext = context;
     //populate();
}

@Override
protected boolean onTap(int index){
    /*OverlayItem item = mOverlays.get(index);
    AlertDialog.Builder dialog = new AlertDialog.Builder(mContext);
    dialog.setTitle(\"Test\");
    dialog.setMessage(\"Test\");
    dialog.show();*/
    Toast.makeText(mContext, \"The int caused te problem\", Toast.LENGTH_SHORT).show();
    return true;
}

public void addOverlays(OverlayItem overlay){
    mOverlays.add(overlay);
    populate();
}

@Override
protected OverlayItem createItem(int i) {
    // TODO Auto-generated method stub

    return mOverlays.get(i);
}

@Override
public int size() {
    // TODO Auto-generated method stub
    return mOverlays.size();
}

}

private String idValue;

@Override
protected boolean isRouteDisplayed() {
    // TODO Auto-generated method stub
    return false;
}

@Override 
protected void onCreate(Bundle icicle){

    super.onCreate(icicle);
    setContentView(R.layout.mapview);

    MapView mapview = (MapView)findViewById(R.id.mapview);
    mapview.setBuiltInZoomControls(true);

    idValue = getIntent().getStringExtra(\"idValue\");

    List<Overlay> mapOverlays = mapview.getOverlays();
    Drawable drawable = this.getResources().getDrawable(R.drawable.androidmarker);
    MapItemizedOverlays itemizedOverlays = new MapItemizedOverlays(drawable);

    GeoPoint point = new GeoPoint(19240000,-99120000);
    OverlayItem overlayitem = new OverlayItem(point, \"Hola, Mundo!\", \"I\'m in Mexico City!\");

    itemizedOverlays.addOverlays(overlayitem);
    mapOverlays.add(itemizedOverlays);

    Toast.makeText(getBaseContext(), idValue, Toast.LENGTH_SHORT).show();
}