Qt Application with layout's, QPushButton and QGraphicsItem

Question

I am trying to draw various shapes like rectangle, ellipse, text etc uisng QGraphicsView and QGraphicsScene. For that I am trying to create an interface where there will be a vertical layout and besides that there will be few buttons. On clicking those buttons, I can show various QGraphicsItem's on screen. I want to create this interface programatically but not using ui.
I tried and ended up this way.

I wanted buttons on the right side and verticalLayout on the left side and both should filled up the whole screen.
Here is my current implementation :

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

    QGraphicsScene* scene = new QGraphicsScene(this);

    QGraphicsView* view = new QGraphicsView(this);
    view->setScene(scene);

    QWidget* mainWidget = new QWidget(this);
    QHBoxLayout *mainLayout = new QHBoxLayout();

    QVBoxLayout *buttonLayout = new QVBoxLayout();
    QVBoxLayout *vlayout2 = new QVBoxLayout();

    vlayout2->addWidget(view);

    QPushButton *btn1 = new QPushButton("Rectangle");
    btn1->setSizePolicy( QSizePolicy::Expanding, QSizePolicy::Preferred );
    
    QPushButton *btn2 = new QPushButton("Ellipse");
    btn2->setSizePolicy( QSizePolicy::Expanding, QSizePolicy::Preferred );
    
    QPushButton *btn3 = new QPushButton("Text");
    btn3->setSizePolicy( QSizePolicy::Expanding, QSizePolicy::Preferred );

    buttonLayout->addWidget(btn1);
    buttonLayout->addWidget(btn2);
    buttonLayout->addWidget(btn3);
    buttonLayout->addStretch();

    mainLayout->addLayout(buttonLayout);
    mainLayout->addLayout(vlayout2);

    mainWidget->setLayout(mainLayout);
}    

Can anyone guide me ?