title: Wagtail 0.8.6 - admin - NoneType' object has no attribute 'allowed_subpage_types'
body: <p>I'm using Wagtail 0.8.6 with Django 1.7.7, when I try to access to a specific page in the admin:</p>
  
  <pre><code>http://127.0.0.1:8020/admin/pages/4/
  </code></pre>
  
  <p>I get the following error:</p>
  
  <pre><code>AttributeError at /admin/pages/4/
  'NoneType' object has no attribute 'allowed_subpage_types'
  </code></pre>
  
  <p>this is the traceback:</p>
  
  <pre><code>Environment:
  
  
  Request Method: GET
  Request URL: http://127.0.0.1:8020/admin/pages/4/
  
  Django Version: 1.7.7
  Python Version: 2.7.3
  Installed Applications:
  ('django.contrib.auth',
   'django.contrib.contenttypes',
   'django.contrib.sessions',
   'django.contrib.messages',
   'django.contrib.staticfiles',
   'raven.contrib.django.raven_compat',
   'compressor',
   'taggit',
   'modelcluster',
   'django.contrib.admin',
   'preventconcurrentlogins',
   'django_medusa',
   'wagtail.wagtailcore',
   'wagtail.wagtailadmin',
   'wagtail.wagtaildocs',
   'wagtail.wagtailsnippets',
   'wagtail.wagtailusers',
   'wagtail.wagtailimages',
   'wagtail.wagtailembeds',
   'wagtail.wagtailsearch',
   'wagtail.wagtailredirects',
   'wagtail.contrib.wagtailmedusa',
   'myapp',
   'django_extensions')
  Installed Middleware:
  ('django.middleware.common.CommonMiddleware',
   'django.contrib.sessions.middleware.SessionMiddleware',
   'django.middleware.csrf.CsrfViewMiddleware',
   'django.contrib.auth.middleware.AuthenticationMiddleware',
   'django.contrib.messages.middleware.MessageMiddleware',
   'django.middleware.clickjacking.XFrameOptionsMiddleware',
   'preventconcurrentlogins.middleware.PreventConcurrentLoginsMiddleware',
   'wagtail.wagtailcore.middleware.SiteMiddleware',
   'wagtail.wagtailredirects.middleware.RedirectMiddleware')
  
  
  Template error:
  In template /home/vagrant/venv/local/lib/python2.7/site-packages/wagtail/wagtailadmin/templates/wagtailadmin/pages/list.html, error at line 265
     'NoneType' object has no attribute 'allowed_subpage_types'
     255 :                         {% if allow_navigation %}
  
  
     256 :                             {% if moving %}
  
  
     257 :                                 {% if page.can_descend %}
  
  
     258 :                                     &lt;a href="{% url 'wagtailadmin_pages_move_choose_destination' page_to_move.id page.id %}" class="icon text-replace icon-arrow-right navigate-pages" title="{% blocktrans with title=page.title %}Explorer subpages of '{{ title }}'{% endblocktrans %}"&gt;{% trans 'Explore' %}&lt;/a&gt;
  
  
     259 :                                 {% endif %}
  
  
     260 :                             {% elif choosing %}
  
  
     261 :                                 {% if page.can_descend %}
  
  
     262 :                                     &lt;a href="{% url 'wagtailadmin_choose_page_child' page.id %}?{{ querystring }}" class="icon text-replace icon-arrow-right navigate-pages" title="{% blocktrans with title=page.title %}Explorer subpages of '{{ title }}'{% endblocktrans %}"&gt;{% trans 'Explore' %}&lt;/a&gt;
  
  
     263 :                                 {% endif %}
  
  
     264 :                             {% else %}
  
  
     265 :                                  {% if page.is_navigable %} 
  
  
     266 :                                     &lt;a href="{% url 'wagtailadmin_explore' page.id %}" class="icon text-replace icon-arrow-right" title="{% blocktrans with title=page.title %}Explore child pages of '{{ title }}'{% endblocktrans %}"&gt;{% trans "Explore" %}&lt;/a&gt;
  
  
     267 :                                 {% elif page_perms.can_add_subpage %}
  
  
     268 :                                     &lt;a href="{% url 'wagtailadmin_pages_add_subpage' page.id %}" class="icon text-replace icon-plus-inverse" title="{% blocktrans with title=page.title %}Add a child page to '{{ title }}'{% endblocktrans %}"&gt;{% trans 'Add subpage' %}&lt;/a&gt;
  
  
     269 :                                 {% endif %}
  
  
     270 :                             {% endif %}
  
  
     271 :                         {% endif %}
  
  
     272 :                      &lt;/td&gt;
  
  
     273 :                 &lt;/tr&gt;
  
  
     274 :             {% endfor %}
  
  
     275 :         {% else %}
  
  
  Traceback:
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/core/handlers/base.py" in get_response
    111.                     response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/contrib/auth/decorators.py" in _wrapped_view
    21.                 return view_func(request, *args, **kwargs)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/wagtail/wagtailadmin/views/pages.py" in index
    65.         'pages': pages,
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/shortcuts.py" in render
    50.     return HttpResponse(loader.render_to_string(*args, **kwargs),
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/loader.py" in render_to_string
    178.         return t.render(context_instance)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/base.py" in render
    148.             return self._render(context)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/base.py" in _render
    142.         return self.nodelist.render(context)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/base.py" in render
    844.                 bit = self.render_node(node, context)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/debug.py" in render_node
    80.             return node.render(context)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/loader_tags.py" in render
    126.         return compiled_parent._render(context)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/base.py" in _render
    142.         return self.nodelist.render(context)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/base.py" in render
    844.                 bit = self.render_node(node, context)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/debug.py" in render_node
    80.             return node.render(context)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/loader_tags.py" in render
    126.         return compiled_parent._render(context)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/base.py" in _render
    142.         return self.nodelist.render(context)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/base.py" in render
    844.                 bit = self.render_node(node, context)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/debug.py" in render_node
    80.             return node.render(context)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/loader_tags.py" in render
    126.         return compiled_parent._render(context)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/base.py" in _render
    142.         return self.nodelist.render(context)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/base.py" in render
    844.                 bit = self.render_node(node, context)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/debug.py" in render_node
    80.             return node.render(context)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/loader_tags.py" in render
    65.                 result = block.nodelist.render(context)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/base.py" in render
    844.                 bit = self.render_node(node, context)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/debug.py" in render_node
    80.             return node.render(context)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/loader_tags.py" in render
    65.                 result = block.nodelist.render(context)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/base.py" in render
    844.                 bit = self.render_node(node, context)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/debug.py" in render_node
    80.             return node.render(context)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/loader_tags.py" in render
    150.                 return template.render(context)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/base.py" in render
    148.             return self._render(context)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/base.py" in _render
    142.         return self.nodelist.render(context)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/base.py" in render
    844.                 bit = self.render_node(node, context)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/debug.py" in render_node
    80.             return node.render(context)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/defaulttags.py" in render
    312.                 return nodelist.render(context)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/base.py" in render
    844.                 bit = self.render_node(node, context)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/debug.py" in render_node
    80.             return node.render(context)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/defaulttags.py" in render
    201.                             nodelist.append(node.render(context))
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/defaulttags.py" in render
    312.                 return nodelist.render(context)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/base.py" in render
    844.                 bit = self.render_node(node, context)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/debug.py" in render_node
    80.             return node.render(context)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/defaulttags.py" in render
    312.                 return nodelist.render(context)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/base.py" in render
    844.                 bit = self.render_node(node, context)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/debug.py" in render_node
    80.             return node.render(context)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/defaulttags.py" in render
    305.                     match = condition.eval(context)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/defaulttags.py" in eval
    898.         return self.value.resolve(context, ignore_failures=True)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/base.py" in resolve
    596.                 obj = self.var.resolve(context)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/base.py" in resolve
    734.             value = self._resolve_lookup(context)
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/base.py" in _resolve_lookup
    788.                             current = current()
  File "/home/vagrant/venv/local/lib/python2.7/site-packages/wagtail/wagtailcore/models.py" in can_add_subpage
    1269.         if not self.page.specific_class.allowed_subpage_types():  # this page model has an empty subpage_types list, so no subpages are allowed
  
  Exception Type: AttributeError at /admin/pages/4/
  Exception Value: 'NoneType' object has no attribute 'allowed_subpage_types'
  </code></pre>
  
  <p>before the upgrade, with Wagtail 0.5 and Django 1.6.5 was working.
  Any idea of what can cause that?</p>