Problem mit "alertDialog nach Klick auf einem Button"

// package und importe

public class Main extends Activity 
{
    public Button delete;
    
    public Button break;
    
    public void onCreate(Bundle saveInstanceState) 
    {
        super.onCreate(saveInstanceState);
        setContentView(R.layout.main);
        
        delete = (Button) findViewById(R.id.DeleteButton);
        break = (Button) findViewById(R.id.BreakButton);
        
        delete.setOnClickListener(new View.OnClickListener()
        {
            public void onClick(View v)
            {
                showAlert();
            }
        });
        
        break.setOnClickListener(new View.OnClickListener()
        {
            public void onClick(View v) 
            {
                setContentView(R.layout.startPage);
            }
        });
    }
    
    protected void showAlert()
    {
        AlertDialog.Builder builder = new AlertDialog.Builder(
                DriveStatusManager.this);
        builder.setMessage("Wollen Sie wirklich löschen?").setCancelable(
                false).setPositiveButton("Ja",
                new DialogInterface.OnClickListener() {
                    public void onClick(DialogInterface dialog, int id) {
                        Main.this.finish();
                    }
                }).setNegativeButton("Nein",
                new DialogInterface.OnClickListener() {
                    public void onClick(DialogInterface dialog, int id) {
                        dialog.cancel();
                   }
               });
        builder.create();
    }
}