Android : spinner element to listview on click app crash(android)

so im a beginner in android/java programming and im trying to do the following: the selected item from my spinner should go in a listview when I click a button.So far I did this,eclipse doesnt show any errors or warnings,but my app crash when I try to launch it:

public class MainActivity extends ActionBarActivity {


    Button button3;
    Button button2;
    Button button4;
    ListView listView1;
    Spinner s1;
    String text;

    ArrayAdapter<String> adapter;
    ArrayList<String> list;
    int itemPos;

     @Override
        protected void onCreate(Bundle savedInstanceState) {

            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);

            button3 = (Button) findViewById(R.id.button3); 
            button4 = (Button) findViewById(R.id.button4); 
            button2 = (Button) findViewById(R.id.button2);
            s1 = (Spinner) findViewById(R.id.spinner1);
            listView1 = (ListView) findViewById(R.id.listView1);



            adapter = new ArrayAdapter<String>(this,
                    android.R.layout.simple_list_item_single_choice, list);  //this part here might not be correct
                listView1.setAdapter(adapter);

                button3.setOnClickListener(new View.OnClickListener() {

                    public void onClick(View v) {

                        String text = s1.getSelectedItem().toString();
                        adapter.add(text);    
                    }});


     listView1.setOnItemClickListener(new OnItemClickListener() {
      @Override
      public void onItemClick(AdapterView<?> parent, View view,
        int position, long id) {

        itemPos = position;

      }
    });}}

I dont get why it crash, how to fix that?

thank you!