QStatusBar::showMessage makes prepared QPainter unusable and causes crash

A call of QStatusBar::showMessage makes a previously prepared QPainter unusable, the application aborts in one of the Q_ASSERTs of qpaintengine_raster.cpp:

    QRasterPaintEngineState *s = state();
    Q_ASSERT(s);

To reproduce the bug, extract the attached "qtbug.zip" and qmake/make the contained application "app". In mainwnd.cpp (MainWnd::eventFilter) you find two braches. If you activate the first (#if 1), the bug occurs. If you activate the second (#if 0), the application works.

Note, that the #if 0-case is only seemingly a practicable workaround, because QStatusBar::showMessage might be called anytime from everywhere, e.g. from a central QtMessageHandler.

Additionally consider to replace the Q_ASSERTs (which came in with Qt 5.6.1) by a more graceful error handling.