Android : How to set button setOnClickListener if the button in a list activity

I created a activity that contain a listview and a button

<?xml version="1.0" encoding="utf-8"?>

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
              android:orientation="vertical"
              android:layout_width="match_parent"
              android:layout_height="match_parent">
    <ListView
            android:layout_width="match_parent"
            android:layout_height="200dp"
            android:id="@android:id/list"></ListView>

    <Button
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="Add Task"
            android:id="@+id/btn"></Button>
</LinearLayout>

In the class, I added a OnClickListener to the button, Then the app cannot be started

public class Test2 extends ListActivity {
    String[] list={"1","2","3"};
    Button btn;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        btn = (Button) findViewById(R.id.btn);

        btn.setOnClickListener(new Button.OnClickListener() {
                                            @Override
                                            public void onClick(View v) {

                                            }
                                        });

                setListAdapter(new ArrayAdapter<String>(this,
                        android.R.layout.simple_list_item_1, list));
    }
}

If I remove this

        btn.setOnClickListener(new Button.OnClickListener() {
                                            @Override
                                            public void onClick(View v) {

                                            }
                                        });

Then the app is completely work, but the button cannot handle event

How to add the listener to the button when the class extends ListActivity?