如何仅在TextField和选择都有值时启用按钮

import tkinter as tk

def check_fields():
    if textfield.get() and dropdown.get():
        button.config(state=tk.NORMAL)
    else:
        button.config(state=tk.DISABLED)

root = tk.Tk()

# 创建TextField和选择框
textfield = tk.Entry(root)
textfield.pack()

dropdown = tk.StringVar(root)
dropdown.set("请选择")
dropdown_menu = tk.OptionMenu(root, dropdown, "选项1", "选项2", "选项3")
dropdown_menu.pack()

# 创建按钮
button = tk.Button(root, text="提交", state=tk.DISABLED)
button.pack()

# 监听TextField和选择框的值变化
textfield.bind("<KeyRelease>", lambda event: check_fields())
dropdown.trace("w", lambda *args: check_fields())

root.mainloop()