tidak bisa membuka aktiviti baru ketika menekan tombol ok

private void btnwindows() {
    final String[] listItems = {"one", "two", "three"};

    AlertDialog.Builder builder = new AlertDialog.Builder(MainActivity.this);
    builder.setTitle("Choose item");

    int checkedItem = 0; //this will checked the item when user open the dialog
    builder.setSingleChoiceItems(listItems, checkedItem, new DialogInterface.OnClickListener() {
        @Override
        public void onClick(DialogInterface dialog, int which) {
            switch (which) {
                case 0:
                    Intent intent = new Intent(MainActivity.this , MainActivity2.class);
                    startActivity(intent);
                    break;
                case 1:
                    Intent intent1 = new Intent(MainActivity.this, MainActivity3.class);
                    startActivity(intent1);
                    break;
                case 2:
                    Intent intent2 = new Intent(MainActivity.this, MainActivity4.class);
                    startActivity(intent2);
                    break;
                default:
            }

        }
    });

    builder.setPositiveButton("Done", new DialogInterface.OnClickListener() {
        @Override
        public void onClick(DialogInterface dialog, int which) {
            dialog.dismiss();
        }
    });

    AlertDialog dialog = builder.create();
    dialog.show();
}