Preoblem with setQuitOnLastWindowClosed (false); and QMainWindow

[ivi2501]

Hi for all.
Now I develop sytem "Task Manager system " based on website + xmlrpc + windows software.

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Some months ago I develop simple software:

QApplication a(argc, argv);
a.setQuitOnLastWindowClosed ( false);

QApplication a(argc, argv);
a.setQuitOnLastWindowClosed ( false);

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Main window based on QWidget

But now I use QMainWindow.

QApplication a(argc, argv);
a.setQuitOnLastWindowClosed (false);
mainWindow = new Gefest(config);
mainWindow->show();

QApplication a(argc, argv);
a.setQuitOnLastWindowClosed (false);
mainWindow = new Gefest(config);
mainWindow->show();

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When I click close on MainWindow window not closed but hide to systemtray.
When I close subwindow software closed.
Please help.
setQuitOnLastWindowClosed not work with QMainWindow?