滚动列表框后，tkinter Listbox.bbox方法返回None

import tkinter as tk

def scroll_listbox(event):
    listbox.yview_scroll(1, "units")  # 向下滚动一个单位

def get_item_bbox(event):
    listbox.update_idletasks()  # 更新列表框的布局
    index = listbox.curselection()[0]
    bbox = listbox.bbox(index)
    if bbox is None:
        print("滚动后，bbox方法返回None")
    else:
        print("滚动后，项目的位置信息为：", bbox)

root = tk.Tk()

scrollbar = tk.Scrollbar(root)
scrollbar.pack(side=tk.RIGHT, fill=tk.Y)

listbox = tk.Listbox(root, yscrollcommand=scrollbar.set)
listbox.pack(side=tk.LEFT, fill=tk.BOTH)

scrollbar.config(command=listbox.yview)

for i in range(50):
    listbox.insert(tk.END, "项目" + str(i+1))

listbox.bind("<MouseWheel>", scroll_listbox)
listbox.bind("<Button-1>", get_item_bbox)

root.mainloop()