How do I create popup window with countdown timer (Python kivy)

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Python

I need to show a popup window with a countdown timer. I have the popup window showing but cant access widgets inside of it while it is open.

From code
App.get_running_app().ids.labUSB.text= "Pathfound"
gives an error and I dont think this is correct way anyway.

Any help please?

class USBPopup(Popup):
    if os.path.ismount(logDir):
        #App.get_running_app().ids.labUSB.text= "Pathfound"
        print("Pathfound")
    else:
        print("Path not found")
    pass

kv file

<USBPopup>:
    labUSB: labUSB
    
    size_hint: .5, .5
    auto_dismiss: False

    #border: (5, 5, 5, 5)
    title: ""                 # <<<<<<<<
    separator_height: 0       # <<<<<<<<
    GridLayout:
        cols: 1
        size: root.width, root.height
        MyTLab:
            text: "Copying log file to USB ..."
            underline: True
            background_color: 1, 1, 1, 1
        MyTLab:
            id:labUSB
            text: "please wait ..."
            underline: True
            background_color: 1, 1, 1, 1
        MyBut:
            text: "Close"
            font_size: 20
            size_hint: .2, .8
            on_press: root.dismiss()

What I have tried:

as above - Need to change labUSB.text while Popup is open.