Wagtail 4.1, 'NoneType' object has no attribute '_inc

I am trying to integrate wagtail into an existing django project, but I am getting the above error when I add a child page.

In my settings I have followed the instructions so:

INSTALLED_APPS = [
    'django.contrib.admin',
    'django.contrib.auth',
    'django.contrib.contenttypes',
    'django.contrib.sessions',
    'django.contrib.messages',
    'django.contrib.staticfiles',
    'django.contrib.humanize',
    'whitenoise.runserver_nostatic',
    'home',
    'news',
    'wagtail.contrib.forms',
    'wagtail.contrib.redirects',
    'wagtail.embeds',
    'wagtail.sites',
    'wagtail.users',
    'wagtail.snippets',
    'wagtail.documents',
    'wagtail.images',
    'wagtail.search',
    'wagtail.admin',
    'wagtail',
    'modelcluster',
]


MIDDLEWARE = [
    'django.middleware.security.SecurityMiddleware',
    'django_permissions_policy.PermissionsPolicyMiddleware',
    'django.contrib.sessions.middleware.SessionMiddleware',
    'django.middleware.common.CommonMiddleware',
    'corsheaders.middleware.CorsMiddleware',
    'django.middleware.csrf.CsrfViewMiddleware',
    'django.contrib.auth.middleware.AuthenticationMiddleware',
    'django.contrib.messages.middleware.MessageMiddleware',
    'django.middleware.clickjacking.XFrameOptionsMiddleware',
    'wagtail.contrib.redirects.middleware.RedirectMiddleware',
]

my project urls:

from django.urls import include, path
from django.conf import settings
from django.conf.urls.static import static
from django.urls import reverse
from wagtail.admin import urls as wagtailadmin_urls
from wagtail import urls as wagtail_urls
from wagtail.documents import urls as wagtaildocs_urls


urlpatterns = [
    path('', include('home.urls')),
    path('news/', include('news.urls')),
    path('cms/', include(wagtailadmin_urls)),
    path('documents/', include(wagtaildocs_urls)),
    path('news/', include(wagtail_urls)),
]   static(settings.MEDIA_URL, document_root=settings.MEDIA_ROOT)

my news urls:

from django.urls import path
from .views import news_article_detail

urlpatterns = [
    path('news/<int:pk>/', news_article_detail, name='news_article_detail'),
]

in my news view:

from django.shortcuts import render
from .models import NewsArticlePage
#import pdb; pdb.set_trace()

def news_article_detail(request, pk):
    news_article = NewsArticlePage.objects.get(pk=pk)
    return render(request, 'news/news_page.html', {'news_article': news_article})

and my model:

from django.db import models
from wagtail.core.models import Page
from wagtail.admin.edit_handlers import FieldPanel
from wagtail.core.fields import RichTextField

class NewsArticlePage(Page):
    article_body = RichTextField(blank=True)

    content_panels = Page.content_panels   [
        FieldPanel('article_body'),
    ]
    
    template = "news/news_page.html"

and in my template:

{% extends "base.html" %}

{% block content %}
<h1>{{ page.title }}</h1>
{{ page.article_body|richtext }}
{% endblock %}

I have also added the root page, made sure that the url's are pointing to the correct place, made migrations & migrated but still I get the same error. So when I go to localhost/cms in the backend I have a page set up, when I go to add a child page I get the error, I tried the python manage.py fixtree but I still get the error, here is the stack trace:

nvironment:

Request Method: POST Request URL: http://127.0.0.1:7000/cms/pages/add/news/newsarticlepage/2/

Django Version: 3.2.4
Python Version: 3.11.1
Installed Applications:
['django.contrib.admin',
 'django.contrib.auth',
 'django.contrib.contenttypes',
 'django.contrib.sessions',
 'django.contrib.messages',
 'django.contrib.staticfiles',
 'django.contrib.humanize',
 'whitenoise.runserver_nostatic',
 'accounts',
 'compressor',
 'corsheaders',
 'home',
 'news',
 'taggit',
 'widget_tweaks',
 'sass_processor',
 'wagtail.contrib.forms',
 'wagtail.contrib.redirects',
 'wagtail.embeds',
 'wagtail.sites',
 'wagtail.users',
 'wagtail.snippets',
 'wagtail.documents',
 'wagtail.images',
 'wagtail.search',
 'wagtail.admin',
 'wagtail',
 'modelcluster']
Installed Middleware:
['django.middleware.security.SecurityMiddleware',
 'django_permissions_policy.PermissionsPolicyMiddleware',
 'django.contrib.sessions.middleware.SessionMiddleware',
 'django.middleware.common.CommonMiddleware',
 'corsheaders.middleware.CorsMiddleware',
 'django.middleware.csrf.CsrfViewMiddleware',
 'django.contrib.auth.middleware.AuthenticationMiddleware',
 'django.contrib.messages.middleware.MessageMiddleware',
 'django.middleware.clickjacking.XFrameOptionsMiddleware',
 'wagtail.contrib.redirects.middleware.RedirectMiddleware']



Traceback (most recent call last):
  File "C:\Users\sesa301575\sites\Django.Azure\venv\Lib\site-packages\django\core\handlers\exception.py", line 47, in inner
    response = get_response(request)
  File "C:\Users\sesa301575\sites\Django.Azure\venv\Lib\site-packages\django\core\handlers\base.py", line 181, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "C:\Users\sesa301575\sites\Django.Azure\venv\Lib\site-packages\django\views\decorators\cache.py", line 44, in _wrapped_view_func
    response = view_func(request, *args, **kwargs)
  File "C:\Users\sesa301575\sites\Django.Azure\venv\Lib\site-packages\wagtail\admin\urls\__init__.py", line 170, in wrapper
    return view_func(request, *args, **kwargs)
  File "C:\Users\sesa301575\sites\Django.Azure\venv\Lib\site-packages\wagtail\admin\auth.py", line 182, in decorated_view
    response = view_func(request, *args, **kwargs)
  File "C:\Users\sesa301575\sites\Django.Azure\venv\Lib\site-packages\django\views\generic\base.py", line 70, in view
    return self.dispatch(request, *args, **kwargs)
  File "C:\Users\sesa301575\sites\Django.Azure\venv\Lib\site-packages\wagtail\admin\views\pages\create.py", line 119, in dispatch
    return super().dispatch(request)
  File "C:\Users\sesa301575\sites\Django.Azure\venv\Lib\site-packages\django\views\generic\base.py", line 98, in dispatch
    return handler(request, *args, **kwargs)
  File "C:\Users\sesa301575\sites\Django.Azure\venv\Lib\site-packages\wagtail\admin\views\pages\create.py", line 132, in post
    return self.form_valid(self.form)
  File "C:\Users\sesa301575\sites\Django.Azure\venv\Lib\site-packages\wagtail\admin\views\pages\create.py", line 148, in form_valid
    return self.save_action()
  File "C:\Users\sesa301575\sites\Django.Azure\venv\Lib\site-packages\wagtail\admin\views\pages\create.py", line 170, in save_action
    self.parent_page.add_child(instance=self.page)
  File "C:\Users\sesa301575\sites\Django.Azure\venv\Lib\site-packages\treebeard\mp_tree.py", line 1089, in add_child
    return MP_AddChildHandler(self, **kwargs).process()
  File "C:\Users\sesa301575\sites\Django.Azure\venv\Lib\site-packages\treebeard\mp_tree.py", line 383, in process
    newobj.path = self.node.get_last_child()._inc_path()

Exception Type: AttributeError at /cms/pages/add/news/newsarticlepage/2/
Exception Value: 'NoneType' object has no attribute '_inc_path'

CodePudding user response：

'NoneType' object has no attribute '_inc_path' usually indicates that the internal fields (path, depth, numchild) defined by the django-treebeard library for keeping track of child counts in the page tree have become inconsistent. It's unclear what might have caused that here, but running ./manage.py fixtree should fix it.