Python 3 kivy ubuntu

Question

I have a simple python kivy source code (only a example) and I execute the code in virtuelenv. The "program" runs without errors. But unfortunately I see not the open window - only the Python Icon on the bottom from my task bar. What I have to do that the windows opens?

# Datei: hello.py

import kivy
kivy.require('1.9.0') # Mindest-Version von Kivy

from kivy.app import App
from kivy.uix.button import Button


class HelloApp(App):

    def build(self):
        return Button(text='Hallo Welt!')

if __name__== "__main__":
    HelloApp().run()

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