Why can't I acces my image's source using Screenmanager and ObjectProperty?

I’d like to access my image’s source by pointing to its ID. When I try it this way without using ScreenManager it returns <kivy.uix.image.Image object at 0x0DA10A78> but using ScreenManager returns “None”
So do you have any idea how to access my image using ScreenManager?

Python code:

from kivy.uix.floatlayout import FloatLayout
from kivy.uix.image import Image


class MainWindow(Screen):

    img1 = ObjectProperty(None)

def __init__(self, **kwargs):
    super(MainWindow, self).__init__(**kwargs)

    self.print_image()

def print_image(self):
    print(self.img1)


class SecondWindow(Screen):
    pass


class WindowManager(ScreenManager):
    pass


Gui = Builder.load_file("my.kv")


class MyApp(App):
    def build(self):
        return Gui


if __name__ == "__main__":
    MyApp().run()

kv code:

WindowManager:
    MainWindow:
    SecondWindow:

<MainWindow>:
    name: "main"
    img1: img1

    Button:
        id: b1
        text: "Submit"

    Image:
        id: img1
        source: "IMG_8681.jpg"


<SecondWindow>:
    name: "second"

    Button:
        text: "Go Back"
        on_release:
            app.root.current = "main"
            root.manager.transition.direction = "right"

Thanks in advance!