如何通过图标按钮KivyMD使用MDDropdownMenu？

from kivy.lang import Builder
from kivy.uix.boxlayout import BoxLayout
from kivymd.app import MDApp
from kivymd.uix.button import MDRaisedButton
from kivymd.uix.menu import MDDropdownMenu
from kivymd.uix.list import OneLineListItem

KV = '''
<MainLayout>:
    orientation: 'vertical'
    padding: dp(20), dp(20)
    spacing: dp(20)

    MDRaisedButton:
        id: dropdown_button
        text: 'Select Option'
        on_release: app.menu.open(self)
'''

class MainLayout(BoxLayout):
    pass

class MyApp(MDApp):
    def build(self):
        self.screen = MainLayout()
        self.menu_items = [
            {
                "viewclass": "OneLineListItem",
                "text": f"Option {i}",
                "on_release": lambda x=f"Option {i}": self.set_item(x),
            }
            for i in range(5)
        ]
        self.menu = MDDropdownMenu(
            caller=self.screen.ids.dropdown_button,
            items=self.menu_items,
            width_mult=4,
        )
        return self.screen

    def set_item(self, text):
        self.screen.ids.dropdown_button.text = text
        self.menu.dismiss()

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