我在屏幕上有一个按钮，我想在按下这个按钮时打开MD卡。我编写的代码与popup相同，但mdcard对象没有“open”属性。如何打开mymdcard

# : import Factory kivy.factory.Factory

<Second>:
    FloatLayout:
        Button:
            text:"Open MDCard"
            size_hint: .3, .07 
            pos_hint: {"x":.07, "y":0.25} 
            on_release: Factory.MyMDCard().open()

<MyMDCard@MDCard>
    orientation: "vertical"
    canvas.before:
        Rectangle:
            pos: self.pos
            size: self.size
            source: 'Blue.png'

    FloatLayout:    
        Button:
            id: 'change'