strange widget problem

// mywidget.h

#ifndef MYWIDGET_H
#define MYWIDGET_H

#include <QWidget>

class MyWidget : public QWidget
{
    Q_OBJECT
public:
    explicit MyWidget(QWidget *parent = 0);

signals:

public slots:

};

#endif // MYWIDGET_H


//---------------------------------------------------------------------------------------


// mywidget.cpp
#include "mywidget.h"

MyWidget::MyWidget(QWidget *parent) :
    QWidget(parent)
{
}


//---------------------------------------------------------------------------------------


// mainwindow.h
#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

namespace Ui {
    class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

private:
    Ui::MainWindow *ui;
};

#endif // MAINWINDOW_H


//---------------------------------------------------------------------------------------


// mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include "mywidget.h"

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
//  QWidget *w = new MyWidget(this);    // this line is ok
    MyWidget *w = new MyWidget(this);   // but this one is not
    w->setGeometry(10,20,100,100);
    w->setStyleSheet("background-color:white;");
    w->show();
}

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