在tkinter (Python3)中将值从子窗口传递到父窗口:意外行为

import tkinter as tk

class ChildWindow(tk.Toplevel):
    def __init__(self, parent):
        super().__init__(parent)
        self.parent = parent
        self.entry = tk.Entry(self)
        self.entry.pack()
        self.button = tk.Button(self, text="确认", command=self.pass_value)
        self.button.pack()

    def pass_value(self):
        value = self.entry.get()
        self.parent.set_value(value)
        self.destroy()

class MainWindow(tk.Tk):
    def __init__(self):
        super().__init__()
        self.value = None
        self.button = tk.Button(self, text="打开子窗口", command=self.open_child_window)
        self.button.pack()

    def open_child_window(self):
        child_window = ChildWindow(self)

    def set_value(self, value):
        self.value = value
        print("从子窗口传递过来的值：", self.value)

if __name__ == "__main__":
    main_window = MainWindow()
    main_window.mainloop()