tkinter variable does not change when changing Checkbutton state

I have a Window wich opens another window askig for the settings. But the BooleanVar I use to get the Checkbutton's state doesn't change. It does however when I call the settingswindow strait from the code without the other window.

This is the minimal code to get the problem: from tkinter import *

class MainWindow():

    def __init__(self, master):

        self.root = master
        SettingsWindow()
        self.root.mainloop()

class SettingsWindow():

    def __init__(self):
        rootSettings = Tk()

        self.rebuild = BooleanVar()
        chkRebuild = Checkbutton(rootSettings, text="rebuild", variable=self.rebuild, command=self.testFunc)
        chkRebuild.pack()

        rootSettings.mainloop()

    def testFunc(self):
        print(self.rebuild.get())

root = Tk()
mainWindow = MainWindow(root)

The output is always 0 when clicking on the checkbutton, evenso the output of BooleanVar is True or False.

What is the difference in calling SettingsWindow() from insite a class or outside? At least I think that is the reason it is not working.

I am using Pyhton3 in case there is a difference.