Widgets must be created in the GUI thread.

[stampede]

If you want to use QtGui, then you have to instantiate QApplication object, QCoreApplication is not enough to use message windows, change this

#include <QtCore/QCoreApplication>
...
QCoreApplication a(argc, argv);

#include <QtCore/QCoreApplication>
...
QCoreApplication a(argc, argv);

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to

#include <QApplication>
...
QApplication a(argc, argv);

#include <QApplication>
...
QApplication a(argc, argv);

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