How to set ui->widget as Qt3DExtras::Qt3DWindow

[sonichy]

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <Qt3DCore/QEntity>
#include <Qt3DExtras/QPhongMaterial>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

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

private:
    Ui::MainWindow *ui;
    Qt3DCore::QEntity *rootEntity;
    Qt3DExtras::QPhongMaterial *material;

private slots:
    void on_actionRender_triggered();

};

#endif // MAINWINDOW_H

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <Qt3DExtras/Qt3DWindow>
#include <Qt3DExtras/QCuboidMesh>
#include <Qt3DCore/QTransform>
#include <Qt3DExtras/QForwardRenderer>
#include <Qt3DRender/QCamera>

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    Qt3DExtras::Qt3DWindow *window;
    Qt3DExtras::Qt3DWindow *window = new Qt3DExtras::Qt3DWindow();
    window->defaultFrameGraph()->setClearColor(QColor(QRgb(0x4d4d9f)));
    ui->widget = window; //here
    rootEntity = new Qt3DCore::QEntity;
    window->setRootEntity(rootEntity);
    window->show();

    // Material
    material = new Qt3DExtras::QPhongMaterial();
    material->setDiffuse(QColor(QRgb(0xbeb32b)));

    Qt3DRender::QCamera *cameraEntity = window->camera();
    cameraEntity->lens()->setPerspectiveProjection(45.0f, 16.0f/9.0f, 0.1f, 1000.0f);
    cameraEntity->setPosition(QVector3D(0, 0, 20.0f));
    cameraEntity->setUpVector(QVector3D(0, 1, 0));
    cameraEntity->setViewCenter(QVector3D(0, 0, 0));
}

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

void MainWindow::on_actionRender_triggered()
{
    Qt3DCore::QEntity *cuboidEntity = new Qt3DCore::QEntity(rootEntity);
    Qt3DExtras::QCuboidMesh *cuboidMesh = new Qt3DExtras::QCuboidMesh();
    cuboidMesh->setXExtent(3);
    cuboidMesh->setYExtent(2);
    cuboidMesh->setZExtent(4);
    Qt3DCore::QTransform *cuboidTransform = new Qt3DCore::QTransform();
    cuboidTransform->setTranslation(QVector3D(1.0f, -1.0f, 1.0f));
    cuboidTransform->setRotationX(30);
    cuboidTransform->setRotationY(40);
    cuboidEntity->addComponent(cuboidMesh);
    cuboidEntity->addComponent(material);
    cuboidEntity->addComponent(cuboidTransform);
}