def quit(self):
        """
        This function quits PlanarRad, checking if PlanarRad is running before.
        """

        """
        Nothing programmed for displaying a message box when the user clicks on the window cross in order to quit.
        """

        if self.is_running == True:
            warning_planarrad_running = QtGui.QMessageBox.warning(self.ui.quit, 'Warning !',
                                                                  "PlanarRad is running. Stop it before quit !",
                                                                  QtGui.QMessageBox.Ok)

        else:
            quit = QtGui.QMessageBox.question(self.ui.quit, 'Quit PlanarRad', "Are you sure to quit ?",
                                              QtGui.QMessageBox.Yes,
                                              QtGui.QMessageBox.No)
            if quit == QtGui.QMessageBox.Yes:
                QtGui.qApp.quit()
This function quits PlanarRad, checking if PlanarRad is running before.