[Python] Why does closing a tkinter child window with `frame.quit` exit my application?...

I have a tkinter application with two windows: A MainWindow that is created on startup, and a ChildWindow which is opened after clicking a button.

The ChildWindow should close itself if the user presses a button. However, when I try calling frame.quit, it terminates the entire application instead.

import tkinter as tk

class ChildWindow:
def __init__(self, master):
self.top = tk.Toplevel(master)
self.frame = tk.Frame(self.top)
self.frame.pack()
# BUG: Clicking "Close" will fully exit application
self.close_button = tk.Button(self.frame, text="Close", command=self.frame.quit)
self.close_button.pack()

class MainWindow:
def __init__(self, master):
self.master = master
self.frame = tk.Frame(self.master)
self.about_button = tk.Button(self.frame, text="Open child window", command=self._open_child_window)
self.about_button.pack()
self.frame.pack()

def _open_child_window(self):
self.about_window = ChildWindow(self.master)

root = tk.Tk()
lf = MainWindow(root)
root.mainloop()


Screenshots:



Why does frame.quit exit my application? How can I close the child window without exiting?

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