Kivy ScreenManager切换屏幕离开按钮

我使用ScreenManager来切换屏幕,但当我切换屏幕时,屏幕上的按钮通常会保持不变。我必须把按钮复制到每个屏幕上。有没有办法在保持按钮不变的情况下切换屏幕?

这是我的应用程序代码:

import (...)


class AppContainer(FloatLayout):
    pass


class NavButtons(BoxLayout):
    pass


class FirstScreen(Screen):
    pass


class SecondScreen(Screen):
    pass


class Screens(ScreenManager):
    pass


class MainApp(App):
    def build(self):
        return AppContainer()

(...)

kv文件如下所示:

#:import BoxLayout kivy.uix.boxlayout.BoxLayout

<AppContainer>:
    NavButtons:
    Screens:
        FirstScreen:
        SecondScreen:
        ThirdScreen:


<NavButtons>:
    orientation:'vertical'
    Button:
        on_press: root.manager.current="first"
        text: 'First'
        pos_hint:{"top": 1, "left": 0}
    Button:
        on_press: root.manager.current="second"
        text: 'Second'
        pos_hint:{"top":0.8, "left": 0}
<Button>:
    size_hint: 0.2, 0.2


<FirstScreen>:
    name: "first"
    Label:
        text: "First Screen"


<SecondScreen>:
    name: "second"
    Label:
        text: "Second Screen"

但这会引发一个错误:AttributeError:“NavButtons”对象没有属性“manager”

有什么想法吗?