버튼을 누른 후 ModalView에서 카운트 다운 타이머를 생성하려면 어떻게합니까?

타이머가있는 모달 뷰를 만들려고합니다. "시작"버튼을 누르면 모달보기가 나타나고 카운트 다운이 시작됩니다. 그러나 valueerror 메시지가 표시됩니다. ValueError : TimerView.timer에 잘못된 형식이 있습니다 (0x0000017AD40D6180 >>>에 main.TimerView 개체가 있음). 코드의 어느 부분이 잘못되었는지 알 수 있습니까? 미리 감사드립니다.

from kivymd.app import MDApp
from kivy.lang import Builder
from kivy.uix.screenmanager import ScreenManager, Screen, FadeTransition, WipeTransition, NoTransition, SlideTransition
from kivymd.theming import ThemeManager
from kivy.properties import StringProperty, NumericProperty
from kivy.uix.modalview import ModalView
from kivymd.uix.label import MDLabel
from kivy.clock import Clock

class TimerView(ModalView):
    number = NumericProperty(15)

    def __init__(self, **kwargs):
        super().__init__(**kwargs)
        self.background = "transparent_image.png"
        self.background_color = [1/3,1/3,1/3,0.8]
        self.auto_dismiss = False
        self.size_hint = (None,None)
        self.size = (150,50)
        timer_countdown = MDLabel(font_style = 'H1', theme_text_color = 'Primary',
                                  text = str(self.number), halign = 'center',
                                  text_color = (1,1,1,1), size_hint_y = None)
        self.add_widget(timer_countdown)

    def decrement_time(self, dt):
        self.number -= 1

    def start(self,*args):
        self.timer = Clock.schedule_interval(self.decrement_time, 1)

    def stop(self):
        Clock.unschedule(self.timer)

class MainScreen(Screen):
    pass

class BeginScreen(Screen):
    pass

class MyScreenManager(ScreenManager):

    def __init__(self,**kwargs):
        super().__init__(**kwargs)
        self.view = TimerView()

    def open_view(self):
        self.view.bind(on_open=self.view.start)
        self.view.open()

main_widget_kv = ('''
#: import ScrollEffect kivy.effects.scroll.ScrollEffect

MyScreenManager:
    BeginScreen:

<BeginScreen>:
    begin_button:begin_button
    name: "begin"
    canvas.before:
        Color: 
            rgb: .1, .1, .1
    FloatLayout:
        id: begin_layout
        Button:
            id: begin_button
            text: 'Begin'
            font_size: 24
            on_press: app.root.open_view()
            size_hint: (.4,.25)
            pos_hint: {"center_x":.5, "center_y":.2}
            color: [0,0,0,1]


''')

class TestApp(MDApp):

    def __init__(self,**kwargs):
        self.theme_cls.theme_style = "Dark"
        self.theme_cls.primary_palette = "Red"
        super().__init__(**kwargs)

    def build(self):
        main_widget = Builder.load_string(main_widget_kv)
        return main_widget


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