title: Can't install django_messages app
body: <p>I tried to install django_messages</p>
  
  <p>I copied the folder in my directory and ran this command:</p>
  
  <pre><code>python manage.py makemigrations
  </code></pre>
  
  <p>The error is this:</p>
  
  <pre><code>C:\Users\Admin\Desktop\sample\enginesearch1&gt;python manage.py runserver
  Performing system checks...
  
  Unhandled exception in thread started by &lt;function check_errors.&lt;locals&gt;.wrapper at 0x0000025D3C50A488&gt;
  Traceback (most recent call last):
    File "C:\Users\Admin\AppData\Local\Programs\Python\Python36\lib\site-packages\django\utils\autoreload.py", line 225, in wrapper
      fn(*args, **kwargs)
    File "C:\Users\Admin\AppData\Local\Programs\Python\Python36\lib\site-packages\django\core\management\commands\runserver.py", line 121, in inner_run
      self.check(display_num_errors=True)
    File "C:\Users\Admin\AppData\Local\Programs\Python\Python36\lib\site-packages\django\core\management\base.py", line 410, in check
      raise SystemCheckError(msg)
  django.core.management.base.SystemCheckError: SystemCheckError: System check identified some issues:
  
  ERRORS:
  django_messages.Message.sender: (fields.E320) Field specifies on_delete=SET_NULL, but cannot be null.
          HINT: Set null=True argument on the field, or change the on_delete rule.
  </code></pre>
  
  <p>What's the problem here and where should i change?</p>