Unable to add QToolButton to QToolbar on QWidget

[rawfool]

I've added a QToolBar to QWidget and added a QToolButton to the QToolBar, but the image isn't showing up. If I use QPushButton, it's showing up any reason?

Widget::Widget(QWidget *parent)
    : QWidget(parent)
{
    oMainLayout = new QVBoxLayout();
    oMainLayout->setMargin(0);
    oToolbar = new QToolBar();
    oMainLayout->addWidget(oToolbar, 0, Qt::AlignTop);
 
    oClose = new QPushButton();
    oToolbar->addWidget(oClose);
    connect(oClose, SIGNAL(clicked()), this, SLOT(onCloseBtnClick()));
    oClose->show();
    this->setLayout(oMainLayout);
}

Widget::Widget(QWidget *parent)
    : QWidget(parent)
{
    oMainLayout = new QVBoxLayout();
    oMainLayout->setMargin(0);
    oToolbar = new QToolBar();
    oMainLayout->addWidget(oToolbar, 0, Qt::AlignTop);

    oClose = new QPushButton();
    oToolbar->addWidget(oClose);
    connect(oClose, SIGNAL(clicked()), this, SLOT(onCloseBtnClick()));
    oClose->show();
    this->setLayout(oMainLayout);
}

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[Closed:] ToolButton is added but doesn't have any border, so it was difficult to recognize. Sorry for the oversight.