如何在kivymd中更改底部导航的默认活动项目？

from kivymd.app import MDApp
from kivymd.uix.bottomnavigation import MDBottomNavigation, MDBottomNavigationItem

class MyApp(MDApp):
    def build(self):
        # 创建底部导航
        bottom_nav = MDBottomNavigation()

        # 创建底部导航的项目
        item1 = MDBottomNavigationItem(text="项目1", icon="icon1.png")
        item2 = MDBottomNavigationItem(text="项目2", icon="icon2.png")
        item3 = MDBottomNavigationItem(text="项目3", icon="icon3.png")

        # 将项目添加到底部导航
        bottom_nav.add_widget(item1)
        bottom_nav.add_widget(item2)
        bottom_nav.add_widget(item3)

        return bottom_nav

class MyApp(MDApp):
    def build(self):
        # ...

        # 设置默认活动项目
        bottom_nav.current = "项目2"

        # ...

        return bottom_nav