无法停止python中的计时器(after和after_cancel)

import tkinter as tk

class App:
    def __init__(self, root):
        self.root = root
        self.timer_id = None
        self.start_button = tk.Button(root, text="Start Timer", command=self.start_timer)
        self.stop_button = tk.Button(root, text="Stop Timer", command=self.stop_timer)
        self.start_button.pack()
        self.stop_button.pack()

    def start_timer(self):
        if self.timer_id is not None:
            self.root.after_cancel(self.timer_id)
        self.timer_id = self.root.after(1000, self.update_label)

    def stop_timer(self):
        if self.timer_id is not None:
            self.root.after_cancel(self.timer_id)
            self.timer_id = None

    def update_label(self):
        print("Timer tick")
        self.timer_id = self.root.after(1000, self.update_label)

root = tk.Tk()
app = App(root)
root.mainloop()