如何在Kivy中删除动态添加的项目

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

class MyApp(App):
    def build(self):
        self.layout = BoxLayout(orientation='vertical')
        return self.layout

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

def add_item():
    new_item = Button(text='New Item')
    app.layout.add_widget(new_item)

class MyApp(App):
    def build(self):
        self.layout = BoxLayout(orientation='vertical')
        add_button = Button(text='Add Item')
        add_button.bind(on_release=add_item)
        self.layout.add_widget(add_button)
        return self.layout

def remove_item(instance):
    app.layout.remove_widget(instance)

def add_item():
    new_item = Button(text='New Item')
    remove_button = Button(text='Remove')
    remove_button.bind(on_release=remove_item)
    new_item.add_widget(remove_button)
    app.layout.add_widget(new_item)