Some of my pages won't preview in the editor

This might not be related directly to coderedcms, but I've subclassed one of the base pages from that project so maybe.

Anyhow, this is my page model:
```
class ArticleIndexPage(CoderedArticleIndexPage):
"""
Shows a list of article sub-pages.
"""

show_cover = models.BooleanField(default=False)

class Meta:
verbose_name = "Article Landing Page"

index_query_pagemodel = "cms.ArticlePage"
template = "coderedcms/pages/article_index_page.html"

body = StreamField(
LOCAL_LAYOUT_STREAMBLOCKS,
null=True,
blank=True,
use_json_field=True,
)

content_panels = CoderedArticleIndexPage.content_panels + [FieldPanel("show_cover")]
```
This page is similar to lots of other pages I've subclassed from coderedcms base pages, but this one won't generate a preview while editing in the Wagtail admin. It says there's a validation error, but there are no red dots indicating anything wrong on the page. I've tried to comment out LOCAL_LAYOUT_STREAMBLOCKS to see if that caused the issues, but it didn't change anything.

Appreciate any tips on how to solve this.

Start with the ArticleIndexPage definition shown in the issue and reproduce its validation error while editing in the Wagtail admin. Compare this subclass with the other subclassed pages that preview successfully, then trace the validation path to determine why no field indicators appear. Done means the cause is documented and this page produces a usable editor preview.

Escrito por el modelo de indexación a partir del texto del issue.