PyQt关闭子窗口问题，无限循环？

from PyQt5.QtWidgets import QApplication, QMainWindow, QPushButton, QWidget

class MainWindow(QMainWindow):
    def __init__(self):
        super().__init__()
        self.setWindowTitle("Main Window")
        
        self.button = QPushButton("Open Sub Window", self)
        self.button.clicked.connect(self.open_sub_window)
        
    def open_sub_window(self):
        self.sub_window = SubWindow()
        self.sub_window.show()
        
class SubWindow(QWidget):
    def __init__(self):
        super().__init__()
        self.setWindowTitle("Sub Window")
        
        self.button = QPushButton("Close", self)
        self.button.clicked.connect(self.close)
        
app = QApplication([])
main_window = MainWindow()
main_window.show()
app.exec_()