没有名称为“ new_screen”的屏幕

Question

我遇到的问题是，按下获取随机彩色屏幕时，它应该添加一个屏幕小部件并切换到名称为“ new_screen”的屏幕。而是给我一个错误，说没有名称为“ new_screen”的屏幕。

from kivy.app import App
from kivy.base import runTouchApp
from kivy.lang import Builder
from kivy.properties import ListProperty
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.screenmanager import ScreenManager, Screen, FadeTransition
import time
import random

class FirstScreen(Screen):
    pass

class SecondScreen(Screen):
    pass

class ColourScreen(Screen):
    colour = ListProperty([1., 0., 0., 1.])

class MyScreenManager(ScreenManager):
    def new_colour_screen(self):
        self.add_widget(ColourScreen(name="new_screen",colour=[random.random() for _ in range(3)] + [1]))
        self.current = "new_screen"

root_widget = Builder.load_string('''
#:import FadeTransition kivy.uix.screenmanager.FadeTransition

MyScreenManager:
    transition: FadeTransition()
    FirstScreen:
    SecondScreen:

<FirstScreen>:
    name: 'first'
    BoxLayout:
        orientation: 'vertical'
        Label:
            text: 'first screen!'
            font_size: 30
        Image:
            source: 'colours.png'
            allow_stretch: True
            keep_ratio: False
        BoxLayout:
            Button:
                text: 'goto second screen'
                font_size: 30
                on_release: app.root.current = 'second'
            Button:
                text: 'get random colour screen'
                font_size: 30
                on_release: app.root.new_colour_screen()

<SecondScreen>:
    name: 'second'
    BoxLayout:
        orientation: 'vertical'
        Label:
            text: 'second screen!'
            font_size: 30
        Image:
            source: 'colours2.png'
            allow_stretch: True
            keep_ratio: False
        BoxLayout:
            Button:
                text: 'goto first screen'
                font_size: 30
                on_release: app.root.current = 'first'
            Button:
                text: 'get random colour screen'
                font_size: 30
                on_release: app.root.new_colour_screen()

<ColourScreen>:
    BoxLayout:
        orientation: 'vertical'
        Label:
            text: 'colour {:.2},{:.2},{:.2} screen'.format(*root.colour[:3])
            font_size: 30
        Widget:
            canvas:
                Color:
                    rgba: root.colour
                Ellipse:
                    pos: self.pos
                    size: self.size
        BoxLayout:
            Button:
                text: 'goto first screen'
                font_size: 30
                on_release: app.root.current = 'first'
            Button:
                text: 'get random colour screen'
                font_size: 30
                on_release: app.root.new_colour_screen()

''')

class ScreenManagerApp(App):
    def build(self):
        return root_widget

ScreenManagerApp().run()