title: None type variable can be "filled" in pygame
body: <p>I am learning pygame and have been following <a href="https://pythonspot.com/snake-with-pygame/" rel="nofollow noreferrer">this tutorial.</a></p>
  
  <p>There seems to be a problem:</p>
  
  <ul>
  <li>I define in the __ init __ a variable -> <code>_display_surf = None</code> ...</li>
  <li>in order to  make this command:  <code>self._display_surf.fill((0,0,0))</code>
  to render a black background.</li>
  </ul>
  
  <p>Nevertheless, I get this error:</p>
  
  <pre><code>Hello from the pygame community. https://www.pygame.org/contribute.html
  Traceback (most recent call last):
    File "snake.py", line 86, in &lt;module&gt;
     theApp.on_execute()
    File "snake.py", line 79, in on_execute
     self.on_render()
    File "snake.py", line 49, in on_render
     self._display_surf.fill((0,0,0))
   AttributeError: 'NoneType' object has no attribute 'fill'
  </code></pre>
  
  <p>Hence my question: <strong>what type should the variable I declare ?</strong></p>