Matplotlib保存按钮在嵌入式Tkinter窗口中不起作用

import tkinter as tk
from matplotlib.figure import Figure
from matplotlib.backends.backend_tkagg import FigureCanvasTkAgg, NavigationToolbar2Tk

root = tk.Tk()

# 创建一个Matplotlib的Figure对象
fig = Figure()

# 在Figure对象上添加一个子图
ax = fig.add_subplot(111)

# 在子图上绘制一些数据
ax.plot([1, 2, 3, 4], [1, 4, 9, 16])

# 创建一个FigureCanvasTkAgg对象，将Figure对象绑定到Tkinter窗口中
canvas = FigureCanvasTkAgg(fig, master=root)
canvas.draw()

# 添加一个Matplotlib的导航工具栏
toolbar = NavigationToolbar2Tk(canvas, root)
toolbar.update()

# 将FigureCanvasTkAgg对象和导航工具栏添加到Tkinter窗口中
canvas.get_tk_widget().pack(side=tk.TOP, fill=tk.BOTH, expand=1)
toolbar.pack(side=tk.BOTTOM, fill=tk.BOTH)

# 手动添加事件循环
root.mainloop()

# 定义保存函数
def save_figure():
    filename = tk.filedialog.asksaveasfilename(defaultextension=".png")
    fig.savefig(filename)

# 添加一个保存按钮到导航工具栏中
save_button = tk.Button(toolbar, text="保存", command=save_figure)
save_button.pack(side=tk.LEFT)