如何在kivy中删除弹出式标题

from kivy.app import App
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.button import Button
from kivy.uix.popup import Popup

class CustomLayout(BoxLayout):
    def __init__(self, **kwargs):
        super(CustomLayout, self).__init__(**kwargs)
        self.orientation = 'vertical'
        self.add_widget(Button(text='Content 1'))
        self.add_widget(Button(text='Content 2'))

class MyApp(App):
    def build(self):
        popup = Popup(title='', content=CustomLayout(), size_hint=(None, None), size=(400, 400))
        return Button(text='Open Popup', on_release=popup.open)

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