Can't use SingleApplication

[AndrzejB]

I try https://github.com/itay-grudev/SingleApplication
I set macro set(QAPPLICATION_CLASS QGuiApplication)
but still error "QWidget: Cannot create a QWidget without QApplication"
is in qwidget.cpp

void QWidgetPrivate::init(QWidget *parentWidget, Qt::WindowFlags f)
{
    Q_Q(QWidget);
    Q_ASSERT_X(q != parentWidget, Q_FUNC_INFO, "Cannot parent a QWidget to itself");

    if (Q_UNLIKELY(!qobject_cast<QApplication *>(QCoreApplication::instance())))
        qFatal("QWidget: Cannot create a QWidget without QApplication");

[JonB]

@AndrzejB
The example, and the error message, is for QApplication. You have put in set(QAPPLICATION_CLASS QGuiApplication). You need QApplication for widget applications. You can see that, and the consequent error message if you do not, in the code you pasted.