qApp->quit() does not work ,when i use qt class as a framework in Final Cut Pro plugIn

im working on a Final Cut Pro 7 plugIn . i need to use some qt classes in FCP plugin project .when FCP is running , FCP plugin use qt class and show qt widget. after i close all Qt windows, i find that the QApplication seems not to quit normally. this cause FCP can not be closed, i clicked "quit " in FCP menu, but i got nothing happen. qApp->exit()/quit() QApplication::exit( 1)/quit() does not work neither.

i also find the Qt Event handling works unnormally

sample code:
QApplication::setAttribute(Qt::AA_MacPluginApplication);
QApplication::setAttribute(Qt::AA_DontUseNativeMenuBar);
QApplication::setQuitOnLastWindowClosed(true);
QApplication(argc, (char **)0, 0);

Qwidget a
a.setWindowModality(Qt::WindowModal);
a.setAttribute(Qt::WA_QuitOnClose);
a.setAttribute (Qt::WA_DeleteOnClose);
a.show();
return QApplication::exec();