MarbleWidget with Qt5 support - example code no longer exits properly

Question

I'm writing a marine navigation app. I am a Qt5 and Marble newbie, but this was working so well, until I hit this block.

It uses MarbleWidget as a way of displaying outputs from the application, closely modelled on an example application supplied with Marble.

When I ported this to Qt5 at recent releases (opensuse), 5.14.0, it stopped working properly - it will no longer exit the application by clicking on the x button in the top right of the title bar.

This behaviour is exhibited by the sample code supplied with Marble as well.

Here is the sample application:

#include <QtWidgets/QApplication>
#include <marble/MarbleWidget.h>
#include <marble/GeoPainter.h>

using namespace Marble;

class MyMarbleWidget : public MarbleWidget
{
public:
    virtual void customPaint(GeoPainter* painter);
};

void MyMarbleWidget::customPaint(GeoPainter* painter)
{
    GeoDataCoordinates home(8.4, 49.0, 0.0, GeoDataCoordinates::Degree);
    painter->setPen(Qt::green);
    painter->drawEllipse(home, 7, 7);
    painter->setPen(Qt::black);
    painter->drawText(home, "Hello Marble!");
}

int main(int argc, char** argv)
{
    QApplication app(argc,argv);
    MyMarbleWidget *mapWidget = new MyMarbleWidget;
    mapWidget->setMapThemeId("earth/openstreetmap/openstreetmap.dgml");
    mapWidget->show();
    return app.exec();
}

and my Makefile is:

all: pre bld post

bld: quick_marble.x

quick_marble.x : quick_marble.cpp
    g++ -g -fpic -I /usr/include/qt5/ -I/usr/include/qt5/QtWidgets \
        -I/usr/include/qt5/QtGui \
        -I/usr/include/qt5/QtCore \
        -o quick_marble.x \
        quick_marble.cpp \
        -L/usr/local/lib64 -lmarblewidget-qt5 -lQt5Core -lQt5Widgets -lQt5Gui

pre :
    rm -f *.x
    rm -f *.list

post : 
    ldd quick_marble.x >quick_marble.modules.list
    echo all done

I have tried some work-arounds, but nothing seems to solve this issue.

I have posted the issue in a marble forum, but it hasn't been looked at for several months.

Any ideas to move forward would be helpful.