How to close the current screen after opening a new javafx

Question

well I have this code that will open a new screen but the old screen is still open:

 if (conn.resultset.next()) {
            MenuPrincipal p = new MenuPrincipal ();

code:

/*
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package semeqapp;

import Conexao.ConnectionFactory;
import MenuPrincipal.MenuPrincipal;
import java.io.IOException;
import java.net.URL;
import java.sql.SQLException;
import java.util.ResourceBundle;
import java.util.logging.Level;
import java.util.logging.Logger;
import javafx.event.ActionEvent;
import javafx.fxml.FXML;
import javafx.fxml.Initializable;
import javafx.scene.control.PasswordField;
import javafx.scene.control.TextField;
import javafx.stage.Stage;
import javax.swing.JOptionPane;

/**
 *
 * @author SpiriT
 */
public class LoginFXController implements Initializable {

    @FXML
    private TextField jLogin;
    @FXML
    private PasswordField jSenha;

    @FXML
    private void handleButtonAction(ActionEvent event) {

    }
    @FXML
    private void validar(ActionEvent event) {
        validaLogin();
    }
    public void validaLogin(){
        try {
            ConnectionFactory conn = new ConnectionFactory();
            conn.getConnection();

            String sql = "SELECT login,senha FROM usuario where login = '" + jLogin.getText() + "' and  senha = '" + jSenha.getText() + "'";
            conn.executeSQL(sql);

            //Se houver resultado, ou seja, se validar o usuario e senha, faça algo.
            if (conn.resultset.next()) {
            MenuPrincipal p = new MenuPrincipal ();

                try {
                    p.start(new Stage());
                } catch (IOException ex) {
                    Logger.getLogger(LoginFXController.class.getName()).log(Level.SEVERE, null, ex);
                }
            } else {
            JOptionPane.showMessageDialog(null, "Senha Invalida");
            }
        }catch(SQLException e){
            System.out.println("Erro: "+e);
        }
    }
    @Override
    public void initialize(URL url, ResourceBundle rb) {
        // TODO
    }    

}