QMessageBox disabled the close button when you setStandardButtons(NoButton)!

[koilin]

I have a QMessageBox, when I did the

QMessageBox myBox = new QMessageBox();
myBox.setStandardButtons(NoButton);
myBox.exec();

QMessageBox myBox = new QMessageBox();
myBox.setStandardButtons(NoButton);
myBox.exec();

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the dialog's close button(on the upright corner of the dialog) will turn to be disable.
How to enable the close button?