Почему при добавлении дочернего лайаута словарь ids пуст?

from kivy.app import App
from kivy.lang import Builder

timer_kv = '''
BoxLayout:
    orientation: 'vertical'
    minute: minute
    second: second
    second_100: second_100
    GridLayout:
        cols: 5
        Label:
            id: minute
            text: '00'
            font_size: 60
        Label:
            text: ':'
            font_size: 60
        Label:
            id: second
            text: '00'
            font_size: 60
        Label:
            text: '.'
            font_size: 60
        Label:
            id: second_100
            text: '00'
            font_size: 60
    ScrollView:
        GridLayout:
            cols: 1
            size_hint_y: 1, None
            height: self.minimum_height
    BoxLayout:
        orientation: 'horizontal'
        Button:
            id: btn_clear
            text: 'Clear'
            size_hint: .3, .3
            on_press: app.clearing(self.parent)
        Button:
            id: btn_start_or_stop
            size_hint: .3, .3
            text: 'Start'
            on_press: app.starting_or_stopping(self.parent)
        Button:
            id: btn_round
            size_hint: .3, .3
            text: 'Round'
            on_press: app.add_round(self.parent)
'''
# self.minimum_height - настройка высоты разметки на высоту дочерних виджетов

class TimerApp(App):
    def build(self):
        return Builder.load_string(timer_kv)

    def clearing(self, instance):
        pass

    def starting_or_stopping(self, instance):
        print(instance.ids)

    def add_round(self, instance):
        pass

if __name__ == '__main__':
    TimerApp().run()