初学QT，不知道下面的程序窗口为什么显示不了pushbutton，求过路人指点,该如何处理

#include "MessageBoxs.h"
#include <QtGui>
MessageBoxs::MessageBoxs(QWidget *parent):QDialog(parent)
{
        setWindowTitle(tr("Custom test~~"));

        QPushButton *PushButton_Custom = new QPushButton("Custom");
        PushButton_Custom->setText(tr("Custom.."));

        QGridLayout *GridLayout = new QGridLayout;
        GridLayout->addWidget(PushButton_Custom,0,1);
        GridLayout->setMargin(10);
        GridLayout->setSpacing(20);

        connect(PushButton_Custom,SIGNAL(clicked()),this,SLOT(Slot_Custom()));
}
MessageBoxs::~MessageBoxs()
{

}
void MessageBoxs::Slot_Custom()
{

}

#ifndef MESSAGEBOX_H
#define MESSAGEBOX_H
#include <QDialog>
class MessageBoxs:public QDialog
{
        Q_OBJECT
public:
        MessageBoxs(QWidget *parent = 0);
        ~MessageBoxs();
public slots:
        void Slot_Custom();
};
#endif

#include <qapplication.h>
#include <QPushButton>
#include "MessageBoxs.h"
int main(int argc,char **argv)
{
        QApplication app(argc,argv);
        //QPushButton PushButton_Custom("hello world!!");
        //PushButton_Custom.show();
        MessageBoxs *MessageBoxs_Custom = new MessageBoxs();
        MessageBoxs_Custom->show();
        return app.exec();
}