Error in QLayout after setFont

Widget::Widget(QWidget *parent)
    : QWidget(parent)
    , ui(new Ui::Widget)
{
    ui->setupUi(this);
    //Android，create 10 buttons，click any button, change the size of the font.
    auto hb = new QHBoxLayout(this);
    for(auto i=9;i<19;i++){
        auto pb = new QPushButton(QString(u8"%1").arg(i),this);
        hb->addWidget(pb);
        connect(pb,&QPushButton::clicked,[&,i]{
            qApp->setFont(QFont(qApp->font().family(),i));
            //setFont(QFont(qApp->font().family(),i));
            //Both QApplication::setFont and QWidget::setFont will have the exception.
        });
    }
}