Black screen with kivy1.11.1 python3.6.8 in venv Ubuntu18.04

Question

The code below produces a blank screen entitled "Test", but that is all. No error messages. What am I doing wrong, please?

#!/usr/bin/env python3
from kivy.app import App
from kivy.uix.widget import Widget
from kivy.uix.label import Label
from kivy.uix.floatlayout import FloatLayout
from kivy.uix.scatter import Scatter
class MyScreen(Widget):
    def __init__(self):
        Widget.__init__(self)
        f = FloatLayout()
        s = Scatter()
        l = Label(text="Hello, World!", font_size=100)
        f.add_widget(s)
        s.add_widget(l)
class TestApp(App):
    def build(self):
        return MyScreen()
if __name__ == "__main__":
    TestApp().run()

Answer 1

f = FloatLayout()
        s = Scatter()
        l = Label(text="Hello, World!", font_size=100)
        f.add_widget(s)
        s.add_widget(l)

These added the widgets to the layouts - but you didn't add the layout to the main App Class

try

self.add_widget(f)