#12818 correct evaluation of message box result info

The help page of int QMessageBox::exec() (​https://doc.qt.io/qt-6/qmessagebox.html#exec) says:
When using a QMessageBox with standard buttons, this function returns a StandardButton value indicating the standard button that was clicked.
When using QMessageBox with custom buttons, this function returns an opaque value; use clickedButton() to determine which button was clicked.