An empty window

I deleted the button command "Register". It helped, but I need it. So basically everything except window dissappeared.

import tkinter as tk
root = tk.Tk()
frame = tk.Frame(root)
label3 = tk.Label(root, text="Username")
label2 = tk.Label(root,text=("Password"))
entry_1 = tk.Entry(root)
entry_2 = tk.Entry(root)
label3.grid(row=0)
label2.grid(row=1)
entry_1.grid(row=0,column=1)
entry_2.grid(row=1,column=1)
worker1 = {'Name':'Passwardo','age':25,'obligations':['create a      program','learn computer Science']}
def Register():
    root.withdraw()
    regwin = tk.Tk()
    framere = tk.Frame(regwin)
    reglab1 = tk.Label(root,text = "Enter your username:")
    reglab2 = tk.Label(root,text = "Enter your pass:")
    reglabag = tk.Label(root,text = "Enter your age")
    regentry3 = tk.Entry(root)
    regentry4 = tk.Entry(root)
    reglab1.grid(row=0)
    reglab2.grid(row=1)
    regentry3.grid(row=0,column=1)
    regentry4.grid(row=1,column=1)
    reglabag.grid(row=2)
    but2 = tk.Button(root,     text='Exit',command=root.update()).grid(row=3, column=1,  pady=4)
    regwin.mainloop()

but1 = tk.Button(root, text='Register',command=Register()).grid(row=3, column=1,  pady=4)







root.mainloop()

I expected a new window, with registration form inside.