Some of my pages won't preview in the editor #575

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.