problema horzontalscrollview

ciao
ho definito il seguente xml :

Codice:

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical" >  
    
<HorizontalScrollView android:layout_width="fill_parent" android:layout_height="wrap_content">
    <LinearLayout
    android:id="@+id/gn"  android:orientation="horizontal"
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content"
    />
</HorizontalScrollView>

</FrameLayout>

ho poi fatto la seguente activity , che riceve 4 immagini da una activity chiamante :

Codice:

public class GalleryActivityN<MotionEvent> extends Activity {

	static TextView mDotsText[];
	private int mDotsCount;
	private LinearLayout mDotsLayout;
	ArrayList<String> url = new ArrayList<String>();
	ArrayList<String> images = new ArrayList<String>();
	String image1 = "";
	String image2 = "";
	String image3 = "";
	String image4 = "";
	// private ProgressDialog dialog;
	
 	private ProgressDialog dialog;
	private ImageView imgView;
	//Context contesto; 

	@Override
	public void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.gallerian);

        
		Intent intent = getIntent();

		image1 =  intent.getStringExtra("image1");
		image2 =  intent.getStringExtra("image2");
		image3 =  intent.getStringExtra("image3");
		image4 =  intent.getStringExtra("image4");

		images.add(image1);
		if (!image2.equals("")) {
    		images.add(image2);
		}
		if (!image3.equals("")) {
    		images.add(image3);
		}
		if (!image4.equals("")) {
    		images.add(image4);
		}

		
		HorizontalScrollView hsw = (HorizontalScrollView) findViewById(R.id.gn);
		
	    for (int i=0; i<images.size(); i++) {     

	    	//String a = images.get(i); 
         	//ImageView img = new ImageView(GalleryActivityN.this);   
         	ImageView img = new ImageView(getBaseContext());   

            //img.setMaxWidth(40);     
            //img.setMaxHeight(40);      
            LayoutParams imageViewLayoutParams 
                         = new LayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);
            img.setLayoutParams(imageViewLayoutParams);
           
            img.setId(i+3000);     
            
            //img.setScaleType("fitXY");    
            img.setImageResource(i);
            hsw.addView(img);
            
            //img.setClickable(true);

            setContentView(hsw);
            
            //hsw.setOnClickListener(viewOnClickListener);
            img.setOnClickListener((OnClickListener) img);

	    } 

        . . . . 

però l'esecuzione va in errore eseguendo l''istruzione :
HorizontalScrollView hsw = (HorizontalScrollView) findViewById(R.id.gn);

che errore faccio ?
grazie
ciao