我想通过按下按钮来更改Kivy Mapview的位置，但收到错误消息

from kivy.app import App
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.button import Button
from kivy.garden.mapview import MapView

class MapApp(App):
    def build(self):
        layout = BoxLayout(orientation='vertical')
        
        # 创建一个按钮
        button = Button(text='Change Map Position')
        button.bind(on_press=self.change_map_position)
        
        # 创建一个Mapview小部件
        mapview = MapView(zoom=11, lat=37.7749, lon=-122.4194)
        
        # 将按钮和Mapview添加到布局中
        layout.add_widget(button)
        layout.add_widget(mapview)
        
        return layout
    
    def change_map_position(self, instance):
        # 修改地图的位置
        mapview = instance.parent.children[1]  # 获取Mapview小部件
        mapview.center_lat = 40.7128  # 设置新的纬度值
        mapview.center_lon = -74.0060  # 设置新的经度值

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