Python 3.x 无效的属性名Python Kivy

我一直在尝试使用kivy创建一个带有某种动画的登录页面。但是，每次我运行脚本时，都会在代码的第17行“size=self.size”中显示“Invalid property name”，之后，我在这里留下了迄今为止的代码请帮助

from kivy.animation import Animation
from kivy.uix import widget
from kivy.uix.screenmanager import ScreenManager
from kivymd.app import MDApp
from kivy.lang import Builder
from kivy.core.window import Window

Window.size = (305, 480)

KV = """
MDScreen:
    name: "LoginPage"
    on_enter:
        app.anim(back)
        app.anim1(back1)
    MDfloatLayout:
        MDfloatLayout:
            id: back
            size_hint_y: .6
            pos_hint: {"center_y": 1.8}
            radius: [0, 0, 0, 40]
            canvas:
                color:
                    rgb: (1, 0, 0, 1)
                rectangle:
                    size = self.size
                    pos = self.pos
                
        MDfloatLayout:
            id: back1
            size_hint_y: .6
            pos_hint: {"center_y": 1.8}
            radius: [0, 0, 0, 40]
            canvas:
                color:
                    rgb: (1, 0, 0, 1)
                Ellipse:
                    size = self.size
                    pos = self.pos
    
    
"""

class LoginApp(MDApp):

    def change_screen(self, name):
        screen_manager.current = name

    def build(self):
        global screen_manager
        screen_manager = ScreenManager()
        screen_manager.add_widget(Builder.load_string(KV))
        return screen_manager

    def anim(self):
        anim = Animation(pos_hint={"center_y": 1.16})
        anim.start(widget)

    def anim1(self):
        anim = Animation(pos_hint={"center_y": .85})
        anim.start(widget)

if __name__ == "__main__":
    app = LoginApp()
    app.run()