QT5编译报错 invalid use of incomplete type 'class Ui :: Widget'

我按照书上说的做一个简单的销售车辆管理的界面，只做了一部分，编译报错invalid use of incomplete type 'class Ui :: Widget'，请高手帮忙看看，应该不复杂，我是新手。卡在这里了。多谢多谢！

widget.h
-----------------------
#ifndef WIDGET_H
#define WIDGET_H

#include <QWidget>
#include <QMenuBar>
#include <QMainWindow>

namespace Ui {
class Widget;
}

class Widget : public QMainWindow
{
Q_OBJECT

public:
explicit Widget(QWidget *parent = 0);
~Widget();
QMenu *salesMgtMenu;
QMenu *pwdMenu;

QAction *salesMgtAction;
QAction *chartAction;
QAction *quitAction;

private slots:
void on_salesMgtMenu_click();
void on_chartMenu_click();
void on_quitMenu_click();

private:
Ui::Widget *ui;
void createMenuBar();
};

void Widget::createMenuBar()
{
salesMgtAction = new QAction (tr("品牌车管理"),this);
chartAction = new QAction (tr("销售统计图表"),this);
quitAction = new QAction (tr("exit"),this);

salesMgtMenu = menuBar()->addMenu(tr("销售管理"));
salesMgtMenu ->addAction(salesMgtAction);
salesMgtMenu ->addAction(chartAction);
salesMgtMenu ->addAction(quitAction);

pwdMenu = menuBar()->addMenu(tr("修改密码"));

connect (salesMgtAction,SIGNAL(triggered()),this, SLOT(on_salesMgtMenu_click()));
connect (chartAction,SIGNAL(triggered()),this, SLOT(on_chartMenu_click()));
connect (quitAction,SIGNAL(triggered()),this, SLOT(on_quitMenu_click()));

}

void Widget::on_salesMgtMenu_click()
{
ui->stackedWidget->setCurrentIndex(0); //此处报错 invalid use of incomplete type 'class Ui :: Widget'

}


void Widget::on_chartMenu_click()
{
ui->stackedWidget->setCurrentIndex(1); //此处报错 invalid use of incomplete type 'class Ui :: Widget'

}

void Widget::on_quitMenu_click()
{
this->close();
}

#endif // WIDGET_H
-------------------------------------------


widget.cpp

#include "widget.h"
#include "ui_widget.h"

Widget::Widget(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::Widget)
{
ui->setupUi(this);
createMenuBar();
}

Widget::~Widget()
{
delete ui;
}
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