Explorer in Wagtail does not show any pages - wagtail

Question

I have two instances of identical code (development and production), and I find this issue happening only on the production. I deployed both to a virgin instance, meaning there was no Wagtail installation before that.
The Wagtail admin shows like I have no pages. I do have 2 pages, and even if I didn't have shouldn't there be a "Welcome to Wagtail Page" (it usually is and it was on my development instance). Furthermore, it looks like there is a portion of a site missing.
That is all I see, no matter how many pages I have created (I create them trough URL)
Edit:
This is the full stack trace in the log
2017-04-21 10:27:08,898 exception 22370 ERROR Internal Server Error: /admin/pages/
Traceback (most recent call last):
File "/opt/..../venv/lib/python3.5/site-packages/django/core/handlers/exception.py", line 42, in inner
response = get_response(request)
File "/opt/..../venv/lib/python3.5/site-packages/django/core/handlers/base.py", line 187, in _get_response
response = self.process_exception_by_middleware(e, request)
File "/opt/..../venv/lib/python3.5/site-packages/django/core/handlers/base.py", line 185, in _get_response
response = wrapped_callback(request, *callback_args, **callback_kwargs)
File "/opt/..../venv/lib/python3.5/site-packages/django/views/decorators/cache.py", line 43, in _cache_controlled
response = viewfunc(request, *args, **kw)
File "/opt/..../venv/lib/python3.5/site-packages/wagtail/wagtailadmin/decorators.py", line 24, in decorated_view
return view_func(request, *args, **kwargs)
File "/opt/..../venv/lib/python3.5/site-packages/wagtail/wagtailadmin/views/pages.py", line 104, in index
'do_paginate': do_paginate,
File "/opt/..../venv/lib/python3.5/site-packages/django/shortcuts.py", line 30, in render
content = loader.render_to_string(template_name, context, request, using=using)
File "/opt/..../venv/lib/python3.5/site-packages/django/template/loader.py", line 68, in render_to_string
return template.render(context, request)
File "/opt/..../venv/lib/python3.5/site-packages/django/template/backends/django.py", line 66, in render
return self.template.render(context)
File "/opt/..../venv/lib/python3.5/site-packages/django/template/base.py", line 208, in render
return self._render(context)
File "/opt/..../venv/lib/python3.5/site-packages/django/template/base.py", line 199, in _render
return self.nodelist.render(context)
File "/opt/..../venv/lib/python3.5/site-packages/django/template/base.py", line 994, in render
bit = node.render_annotated(context)
File "/opt/..../venv/lib/python3.5/site-packages/django/template/base.py", line 961, in render_annotated
return self.render(context)
File "/opt/..../venv/lib/python3.5/site-packages/django/template/loader_tags.py", line 174, in render
return compiled_parent._render(context)
File "/opt/..../venv/lib/python3.5/site-packages/django/template/base.py", line 199, in _render
return self.nodelist.render(context)
File "/opt/..../venv/lib/python3.5/site-packages/django/template/base.py", line 994, in render
bit = node.render_annotated(context)
File "/opt/..../venv/lib/python3.5/site-packages/django/template/base.py", line 961, in render_annotated
return self.render(context)
File "/opt/..../venv/lib/python3.5/site-packages/django/template/loader_tags.py", line 174, in render
return compiled_parent._render(context)
File "/opt/..../venv/lib/python3.5/site-packages/django/template/base.py", line 199, in _render
return self.nodelist.render(context)
File "/opt/..../venv/lib/python3.5/site-packages/django/template/base.py", line 994, in render
bit = node.render_annotated(context)
File "/opt/..../venv/lib/python3.5/site-packages/django/template/base.py", line 961, in render_annotated
return self.render(context)
File "/opt/..../venv/lib/python3.5/site-packages/django/template/loader_tags.py", line 174, in render
return compiled_parent._render(context)
File "/opt/..../venv/lib/python3.5/site-packages/django/template/base.py", line 199, in _render
return self.nodelist.render(context)
File "/opt/..../venv/lib/python3.5/site-packages/django/template/base.py", line 994, in render
bit = node.render_annotated(context)
File "/opt/..../venv/lib/python3.5/site-packages/django/template/base.py", line 961, in render_annotated
return self.render(context)
File "/opt/..../venv/lib/python3.5/site-packages/django/template/loader_tags.py", line 70, in render
result = block.nodelist.render(context)
File "/opt/..../venv/lib/python3.5/site-packages/django/template/base.py", line 994, in render
bit = node.render_annotated(context)
File "/opt/..../venv/lib/python3.5/site-packages/django/template/base.py", line 961, in render_annotated
return self.render(context)
File "/opt/..../venv/lib/python3.5/site-packages/django/template/loader_tags.py", line 70, in render
result = block.nodelist.render(context)
File "/opt/..../venv/lib/python3.5/site-packages/django/template/base.py", line 994, in render
File "/opt/..../venv/lib/python3.5/site-packages/django/template/base.py", line 961, in render_annotated
return self.render(context)
File "/opt/..../venv/lib/python3.5/site-packages/django/template/loader_tags.py", line 174, in render
return compiled_parent._render(context)
File "/opt/..../venv/lib/python3.5/site-packages/django/template/base.py", line 199, in _render
return self.nodelist.render(context)
File "/opt/..../venv/lib/python3.5/site-packages/django/template/base.py", line 994, in render
bit = node.render_annotated(context)
File "/opt/..../venv/lib/python3.5/site-packages/django/template/base.py", line 961, in render_annotated
return self.render(context)
File "/opt/..../venv/lib/python3.5/site-packages/django/template/defaulttags.py", line 314, in render
if match:
File "/opt/..../venv/lib/python3.5/site-packages/django/core/paginator.py", line 112, in __len__
return len(self.object_list)
File "/opt/..../venv/lib/python3.5/site-packages/django/db/models/query.py", line 238, in __len__
self._fetch_all()
File "/opt/..../venv/lib/python3.5/site-packages/django/db/models/query.py", line 1087, in _fetch_all
self._result_cache = list(self.iterator())
File "/opt/..../venv/lib/python3.5/site-packages/wagtail/wagtailcore/query.py", line 380, in specific_iterator
pages = model.objects.filter(pk__in=pks)
AttributeError: 'NoneType' object has no attribute 'objects'

Answer 1

After further investigation, I've found that this error occurs when you've removed a page model definition from your project, but pages of that type still exist. (Wagtail should be able to gracefully handle this situation - the fact that it doesn't is a bug, and I'll file an issue shortly...)
You can track this down by running the following lines on the ./manage.py shell command line:
from django.contrib.contenttypes.models import ContentType
[(ct.id, ct.app_label, ct.model) for ct in ContentType.objects.all() if ct.model_class() is None and ct.app_label != 'wagtailadmin']
Hopefully this will give a result like:
[(2, u'home', u'homepage')]
showing the content type ID, app label and model name of the missing page type.
To get the admin working again, run the following lines (making sure to replace 2 with the content type ID from the result above):
from wagtail.wagtailcore.models import Page
Page.objects.filter(content_type_id=2).update(content_type=ContentType.objects.get(app_label='wagtailcore', model='page'))
This will convert the affected page(s) into Wagtail's basic 'Page' type (which has no content fields other than 'title') - you can then go ahead and delete this page.

Answer 2

For me, #sagman's solution worked with minor changes. Discovered you will get a ModuleNotFoundError if you input wagtail.wagtailcore in the Shell but have wagtail.core in INSTALLED APPS, you need to enter from wagtail.core.models import Page in the Shell instead of wagtail.wagtailcore.models import Page. Everything else works as illustrated by #gasman.