Непонятная проблема c ComboBox QT_Qreator

#include "mainwindow.h"
#include "ui_mainwindow.h"
 
MainWindow::MainWindow(QWidget *parent)
  : QMainWindow(parent)
  , ui(new Ui::MainWindow)
{
  ui->setupUi(this);
  check();
}
 
MainWindow::~MainWindow()
{
  delete ui;
}
 
 
void MainWindow:: check(){
  ui->left_b->show();
  bool left_b = ui->left_b->currentIndex();
  bool right_b = ui->right_b->currentIndex();
 
  bool result = false;
  int operation = ui->operation_b->currentIndex();
 
 
  switch (operation) {
 
  case 1:
       result = left_b && right_b;
        break;
  case 2:
       result = left_b || right_b;
        break;
  case 6:
       result = (left_b && !right_b) || (!left_b && right_b);
        break;
  case 3:
       result = !(left_b && right_b);
        break;
  case 4:
       result = !(left_b || right_b);
        break;
  case 5:
       result = (left_b || !right_b) && (!left_b || right_b);
        break;
  case 0:
       ui->right_b->hide();
       result = !left_b;
        break;
    }
 
    QPalette p = this->palette();
    p.setColor(QPalette::Window, (result ? Qt::green : Qt::red));
    this->setPalette(p);
 
   }
 
 
void MainWindow::on_left_b_currentIndexChanged(int index)
{
  Q_UNUSED(index)
  check();
}
 
void MainWindow::on_right_b_currentIndexChanged(int index)
{
  Q_UNUSED(index)
  check();
}
 
void MainWindow::on_operation_b_currentIndexChanged(int index)
{
  Q_UNUSED(index)
  check();
}

#include "mainwindow.h"
 
#include <QApplication>
 
int main(int argc, char *argv[])
{
  QApplication a(argc, argv);
  MainWindow w;
  w.show();
  return a.exec();
}

#ifndef MAINWINDOW_H
#define MAINWINDOW_H
 
#include <QMainWindow>
#include <QWidget>
 
QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACE
 
class MainWindow : public QMainWindow
{
  Q_OBJECT
  void check();
 
public:
  MainWindow(QWidget *parent = nullptr);
  ~MainWindow();
 
private slots:
  void on_left_b_currentIndexChanged(int index);
 
  void on_right_b_currentIndexChanged(int index);
 
  void on_operation_b_currentIndexChanged(int index);
 
private:
  Ui::MainWindow *ui;
};
#endif // MAINWINDOW_H