获取错误NameError:没有为tkinter登录系统定义名称‘main_screen

import tkinter as tk

def login():
    # 登录逻辑
    pass

def main_screen():
    # 创建主屏幕窗口
    screen = tk.Tk()
    screen.title("登录系统")
    
    # 创建登录表单
    username_label = tk.Label(screen, text="用户名")
    username_label.pack()
    username_entry = tk.Entry(screen)
    username_entry.pack()
    
    password_label = tk.Label(screen, text="密码")
    password_label.pack()
    password_entry = tk.Entry(screen, show="*")
    password_entry.pack()
    
    login_button = tk.Button(screen, text="登录", command=login)
    login_button.pack()
    
    screen.mainloop()

if __name__ == "__main__":
    main_screen()