Widgets become invisible

#include <QApplication>
#include <QGraphicsOpacityEffect>
#include <QMainWindow>
#include <QPropertyAnimation>
#include <QPushButton>
#include <QShowEvent>

class TestWidget: public QWidget
{
public:
    explicit TestWidget(QWidget* parent = 0)
        : QWidget(parent)
    {
        fFadeIn.setStartValue(0);
        fFadeIn.setEndValue(1);
        button->move(100, 100);
        //button->move(10, 10);
        setGraphicsEffect(&fOpEffect);
    }

protected:
    void showEvent(QShowEvent* e) override
    {
        QWidget::showEvent(e);
        if (e->spontaneous()) {
            return;
        }
        fFadeIn.start();
    }

private:
    QPushButton* button = new QPushButton("Test", this);
    QGraphicsOpacityEffect fOpEffect;
    QPropertyAnimation fFadeIn{&fOpEffect, "opacity", &fOpEffect};
};

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    QMainWindow w;
    w.resize(800, 600);
    auto tstWidget = new TestWidget(&w);
    tstWidget->move(0, 0);
    tstWidget->resize(800, 600);
    w.setCentralWidget(tstWidget);
    w.show();
    return a.exec();
}