如何将StringVar传递给另一个类？

from tkinter import *

class FirstClass:
    def __init__(self):
        self.string_var = StringVar()
        self.string_var.set("Hello")

        second_class = SecondClass(self.string_var)
        second_class.print_string_var()

class SecondClass:
    def __init__(self, string_var):
        self.string_var = string_var

    def print_string_var(self):
        print(self.string_var.get())

first_class = FirstClass()