Tkinter function call does not return value

I intend to collect names in lists that will be used in a program, one at a time. The function does return a list and the whole program runs once, but after I call the function UserInput with the button:

ttk.Button(root, text='Add place', command= lambda:UserInput()).pack(ipadx=10, ipady=10)

it remains hanging. Even if I close the window, the program doesn't move to the next line:

print('outside function',  places)

anymore.

The code is:

import tkinter as tk

from tkinter import simpledialog

from tkinter import ttk

root = tk.Tk()

def UserInput():

    places = []

    while True:

        place = simpledialog.askstring('Input', 'Enter name',
                                parent=root)

        if place == None: # 'Cancel'
            continue

        if place == '':  
            break  
        else:
            places.append(place)
    print(places)      

    return(places)

places = (UserInput())  # list of cities to visit

print('outside function',  places)


ttk.Button(root, text='Add place', command= lambda:UserInput()).pack(ipadx=10, ipady=10)

root.mainloop()

Any help would be much appreciated. Thanking you in advance.