为什么Filemanager在android kivymd上不起作用？

我用它来创建文件管理器。

https://kivymd.readthedocs.io/en/latest/components/file-manager/

但是，它不再起作用了。我已经使用buildozer使用android的代码创建了应用程序。但是，当点击按钮时，不会在android上打开文件管理器。我还允许在应用设置中进行存储权限。但是仍然不能打开文件管理器。

from kivy.core.window import Window
from kivy.lang import Builder

from kivymd.app import MDApp
from kivymd.uix.filemanager import MDFileManager
from kivymd.toast import toast


KV = '''
BoxLayout:
    orientation: 'vertical'

    MDToolbar:
        title: "MDFileManager"
        left_action_items: [['menu', lambda x: None]]
        elevation: 10

    FloatLayout:

        MDRoundFlatIconButton:
            text: "Open manager"
            icon: "folder"
            pos_hint: {'center_x': .5, 'center_y': .6}
            on_release: app.file_manager_open()
'''


class Example(MDApp):
    def __init__(self, **kwargs):
        super().__init__(**kwargs)
        Window.bind(on_keyboard=self.events)
        self.manager_open = False
        self.file_manager = MDFileManager(
            exit_manager=self.exit_manager,
            select_path=self.select_path,
            preview=True,
        )

    def build(self):
        return Builder.load_string(KV)

    def file_manager_open(self):
        self.file_manager.show('/')  # output manager to the screen
        self.manager_open = True

    def select_path(self, path):
        '''It will be called when you click on the file name
        or the catalog selection button.

        :type path: str;
        :param path: path to the selected directory or file;
        '''

        self.exit_manager()
        toast(path)

    def exit_manager(self, *args):
        '''Called when the user reaches the root of the directory tree.'''

        self.manager_open = False
        self.file_manager.close()

    def events(self, instance, keyboard, keycode, text, modifiers):
        '''Called when buttons are pressed on the mobile device.'''

        if keyboard in (1001, 27):
            if self.manager_open:
                self.file_manager.back()
        return True


Example().run()

使用kivymd在Android上打开文件管理器的实现是什么？