title: Kivy: Unknown class <WindowManager> Error on simple "Hello World" script?
body: <p>So I was working on different kivy tutorials, all was working well. Then today, I started having an error when running a script that was previously working:</p>
  
  <pre><code>kivy.factory.FactoryException: Unknown class &lt;WindowManager&gt;
  </code></pre>
  
  <p>I tried fiddling around, had no clue what was wrong. Then I tried to run a simple Hello World script:</p>
  
  <pre><code>import kivy
  from kivy.app import App
  from kivy.uix.label import Label
  from kivy.uix.screenmanager import ScreenManager, Screen
  
  
  class MyApp(App):
  
      def build(self):
          return Label(text='Hello world')
  
  
  if __name__ == '__main__':
      MyApp().run()
  </code></pre>
  
  <p>And I still get the exception. Here is what it returns:</p>
  
  <pre><code>   File "C:\Users\Manu\Dropbox\Python\other\Test.py", line 14, in &lt;module&gt;
       MyApp().run()
     File "D:\Progs\Pyton37\lib\site-packages\kivy\app.py", line 828, in run
       self.load_kv(filename=self.kv_file)
     File "D:\Progs\Pyton37\lib\site-packages\kivy\app.py", line 599, in load_kv
       root = Builder.load_file(rfilename)
     File "D:\Progs\Pyton37\lib\site-packages\kivy\lang\builder.py", line 301, in load_file
       return self.load_string(data, **kwargs)
     File "D:\Progs\Pyton37\lib\site-packages\kivy\lang\builder.py", line 399, in load_string
       widget = Factory.get(parser.root.name)(__no_builder=True)
     File "D:\Progs\Pyton37\lib\site-packages\kivy\factory.py", line 131, in __getattr__
       raise FactoryException('Unknown class &lt;%s&gt;' % name)
   kivy.factory.FactoryException: Unknown class &lt;WindowManager&gt;
  </code></pre>
  
  <p>So this is strange, since everything was working well this morning, and I did not install other packages or modules.</p>
  
  <p>I tried pip uninstall kivy, and pip install kivy to get a clean install. Tried rebooting. No luck. And the error is both on PyScripter and PyCharm. </p>
  
  <p>I have no clue what's going on here...
  Thanks,</p>