title: TypeError: add_widget() missing 1 required positional argument: 'screen'
body: <p><code>TypeError: add_widget() missing 1 required positional argument: 'screen'</code></p>
  
  <p>How do I fix this error? or what do I have to change around?
  I'm a bit new to this and there's no other question?    </p>
  
  <pre><code>import kivy
  from kivy.app import App
  from kivy.uix.gridlayout import GridLayout
  from kivy.uix.screenmanager import ScreenManager, Screen
  from kivy.uix.widget import Widget
  from kivy.uix.boxlayout import BoxLayout
  
  from kivy.lang import Builder
  
  kivy.require("1.9.0")
  
  
  class Jan(GridLayout, Screen):
      pass
  
  
  class Feb(GridLayout, Screen):
      pass
  
  
  class WindowManager(ScreenManager):
      pass
  
  
  WindowManager.add_widget(Jan(name='Jan'))
  WindowManager.add_widget(Feb(name='Feb'))
  
  
  class CalenderApp(App):
  
      def build(self):
          return WindowManager()
  
  
  customWidget = CalenderApp()
  
  if __name__ == "__main__":
      customWidget.run()
  </code></pre>
  
  <p>I believe the problem is located in the lines that includes <code>.add_widget</code>.</p>
  
  <pre><code>File "C:/Users/Chris/PycharmProjects/untitled/venv/DayMap.py", line 25, in &lt;module&gt;
      WindowManager.add_widget(Jan(name='Jan'))
  TypeError: add_widget() missing 1 required positional argument: 'screen'
  </code></pre>