I got NullPointerException while i try to run this application for getting data from SQLite database

This is my main activity I can't understand what's the wrong going on.while i try to run this,DDMS show NullPointerException.Please help me for find out the error.

public class MainActivity extends ActionBarActivity implements OnClickListener {

private Button btnAdd, btnView;
private TextView txtView;
private EditText etName;

private DBHelper dbHelper;
// private osList oslist;
private ArrayList<osList> allList;
private osList olist;

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

    btnAdd = (Button) findViewById(R.id.btnAdd);
    btnView = (Button) findViewById(R.id.btnView);
    txtView = (TextView) findViewById(R.id.txtView);
    etName = (EditText) findViewById(R.id.etName);
    btnAdd.setOnClickListener(this);
    btnView.setOnClickListener(this);
}

private void setCustomActionBar() {
    // TODO Auto-generated method stub
    ActionBar actionBar = getSupportActionBar();
    // actionBar.setBackgroundDrawable(new ColorDrawable(
    // com.arifxdroid.listviewdemo.R.color.my_color));
    // actionBar.setDisplayOptions(ActionBar.DISPLAY_SHOW_CUSTOM);
    // View view = getLayoutInflater()
    // .inflate(R.layout.custom_actionbar, null);
    // actionBar.setCustomView(view, new ActionBar.LayoutParams(
    // ActionBar.LayoutParams.MATCH_PARENT,
    // ActionBar.LayoutParams.MATCH_PARENT));
    actionBar.setBackgroundDrawable(new ColorDrawable(R.color.my_color));

}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();
    if (id == R.id.action_settings) {
        return true;
    } else if (id == R.id.action_add) {
        return true;
    }
    return super.onOptionsItemSelected(item);
}

@Override
public void onClick(View v) {

    switch (v.getId()) {

    case R.id.btnAdd:

        String givenName = etName.getText().toString();
        olist = new osList(givenName);
        long inserted = dbHelper.insertOS(olist);
        if (inserted == -1) {
            txtView.setText("Data can't be inserted");
        } else {
            Toast.makeText(getApplicationContext(), "sucessfully added",
                    Toast.LENGTH_SHORT).show();
        }
        break;

    case R.id.btnView:

        allList = dbHelper.getosList();
        if (allList != null || allList.size() == 0) {
            // txtView.setText("NO list found");
            Toast.makeText(getApplicationContext(), "No List found",
                    Toast.LENGTH_SHORT).show();
        } else {
            for (osList l : allList) {
                txtView.append("\n" + l.toString());
            }
        }
        break;

    default:
        break;
    }

}

}

This is SQLite helper class

public class DBHelper extends SQLiteOpenHelper {

public static final String DB_NAME = "ListOfOS";
public static final int VERSION = 1;

public static final String TABLE_NAME = "allOS";
public static final String ID_FIELD = "_ID";
public static final String NAME_FIELD = "osName";
public static final String TABLE_SQL = "CREATE TABLE " + TABLE_NAME + " ("
        + ID_FIELD + " INTEGER PRIMARY KEY AUTOINCREMENT, " + NAME_FIELD
        + " TEXT)";

public DBHelper(Context context) {
    super(context, DB_NAME, null, VERSION);
    // TODO Auto-generated constructor stub
}

@Override
public void onCreate(SQLiteDatabase db) {
    // TODO Auto-generated method stub
    db.execSQL(TABLE_SQL);

}

@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    // TODO Auto-generated method stub

}

public long insertOS(osList oslist) {
    SQLiteDatabase db = this.getWritableDatabase();
    ContentValues value = new ContentValues();

    value.put(NAME_FIELD, oslist.getOsName());
    long inserted = db.insert(TABLE_NAME, "", value);

    db.close();
    return inserted;
}

public ArrayList<osList> getosList() {

    SQLiteDatabase db = this.getReadableDatabase();
    ArrayList<osList> all = new ArrayList<osList>();
    Cursor cursor = db.query(TABLE_SQL, null, null, null, null, null, null);
    if (cursor != null) {
        if (cursor.getCount() > 0) {
            cursor.moveToFirst();
            do {
                String name = cursor.getString(cursor
                        .getColumnIndex(NAME_FIELD));
                osList list = new osList(name);
                all.add(list);
            } while (cursor.moveToNext());
        }
    }
    cursor.close();
    db.close();
    return all;
}

}