设置TextView会抛出nullPointerException

Question

LogCat一直给我一个空指针异常。当我尝试在TextView（tview3）中显示“Numbers”时，问题就开始了。 这是我的代码：

    @Override
public View getView(int position, View convertView, ViewGroup parent) {
    // TODO Auto-generated method stub
    Log.d("context = %s ...", String.valueOf(context));
    Log.d("MyAdapter", "getItemId successful");
    View row = convertView;
    ViewHolder h = null;

    if (row == null) {
        LayoutInflater inflater = (LayoutInflater) context
                .getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        Log.d("MyAdapter", "layout inflater successful");
        row = inflater.inflate(R.layout.single_row, parent, false);
        h = new ViewHolder(row);
        row.setTag(h);
    } else {
        h = (ViewHolder) row.getTag();
    }
    Log.d("MyAdapter", "view row successful");
    Row temp = rowList.get(position);
    h.Course.setText(temp.course);
    h.Points.setText(temp.points);
    h.CW.setText(temp.credit_weight);
    ViewSet vs = new ViewSet();
    vs.settingView();
    return row;

}
}

class ViewSet extends Activity {
    ViewSet() {

    }

    void settingView() {
        setContentView(R.layout.activity_main);
        TextView aTextView = (TextView) findViewById(R.id.tview3);
        aTextView.setText("Numbers");
    }
}