title: django admin keyerror: key not found in form
body: <p>I have a strange problem with my form in django admin.</p>
  <p>I have defined my form, model and modeladmin:</p>
  <pre><code>import datetime
  from cmsplugin_news.admin import NewsAdmin as OldNewsAdmin
  from django import forms
  from django.db import models
  from news.models import News
  
  
  class NewNewsForm(forms.ModelForm):
      class Meta:
          model = News
          fields = ('title', 'is_published', 'pub_date', 'is_featured')
  
  
  class NewsAdmin(OldNewsAdmin ):
      fields = ('title', 'is_published', 'pub_date', 'is_featured')
      form = NewNewsForm
  
  
  class News(models.Model):
      title = models.CharField(_('Title'), max_length=255)
      is_published = models.BooleanField(_('Published'), default=False)
      pub_date = models.DateTimeField(_('Publication date'), default=datetime.datetime.now)
      is_featured = models.BooleanField(_('Featured'), default=False)
  
  admin.site.register(News, NewsAdmin)
  </code></pre>
  <p>All fields in the model seem to appear as well in the admin and form. But when I head to /en/admin/news/news/4/, I'm getting the following KeyError: key &quot;is_featured&quot; not found in form</p>
  <p>Full stacktrace here <a href="https://dpaste.de/9yV3" rel="nofollow noreferrer">https://dpaste.de/9yV3</a></p>
  <p>Any help will be much appreciated. Thanks!</p>