Qt连接槽函数没有效果？

主窗口：QtGuiApplication1

QtGuiApplication1.h

#pragma once

#include <QtWidgets/QMainWindow>
#include "ui_QtGuiApplication1.h"

class QtGuiApplication1 : public QMainWindow
{
    Q_OBJECT

public:
    QtGuiApplication1(QWidget *parent = Q_NULLPTR);
public slots:
    void on_SaleButton_clicked();
    void Receive(int);


private:
    Ui::QtGuiApplication1Class ui;
};

QtGuiApplication1.cpp

#include "QtGuiApplication1.h"
#include "Sale.h"
#include <qmessagebox.h>
QtGuiApplication1::QtGuiApplication1(QWidget* parent)
    : QMainWindow(parent)
{
    ui.setupUi(this);
}
void QtGuiApplication1::on_SaleButton_clicked()//跳转到收银模块
{
    auto sale=new Sale();
    connect(sale, SIGNAL(Send(int)), this, SLOT(Receive(int)));//连接槽函数
    sale->show();
}

void QtGuiApplication1::Receive(int a)
{
    QMessageBox messageBox(QMessageBox::Information, "提示","创建了子窗口", QMessageBox::Yes, NULL);
}

子窗口Sale

Sale.h

#pragma once

#include "ui_Sale.h"
#include <QtWidgets/QMainWindow>

class Sale : public QMainWindow
{
    Q_OBJECT

public:
    Sale(QWidget* parent = Q_NULLPTR);
signals:
    void Send(int);
private:
    Ui::Sale ui;
};

Sale.cpp

#include "Sale.h"
Sale::Sale(QWidget* parent ):QMainWindow(parent)
{
    ui.setupUi(this);
    emit Send(1);
}

我的想法是点击主窗体的SaleButton按钮显示子窗口，同时主窗口弹出对话框，但是能显示子窗口，并没有弹出对话框，槽函数是无效的

void QtGuiApplication1::on_SaleButton_clicked()//跳转到收银模块
{
    auto sale=new Sale();   // 在Sale的构造函数中emit Send(1);这时信号槽都还没有绑定
    connect(sale, SIGNAL(Send(int)), this, SLOT(Receive(int)));//连接槽函数
    sale->show();
}