[Error] Creating default object from empty value

Having freshly installed and configured the plugin (copied the yaml in /config/plugin), I have this critkey error :


My setup is :

- A regular page "_commentaires.fr.md_" with, in frontmatter :
```
---
title: Commentaires
visible: true
---
```

- A template _commentaires.html.twig_, with :

```
{% extends 'partials/base.html.twig' %}
{% block content %}
{% set tag = page.header.user_htmltag|defined('div') %}
<{{ tag }} class="{{ page.header.user_classes }}">
{{ page.content }}
{% include 'partials/comments.html.twig' with {'page': page} %}

{% endblock %}
```

I enabled the plugin for the pages :

```
enable_on_routes:
- '/blog'
- '/critiques'
- '/commentaires'
```

Reproduce the error with the /config/plugin YAML, the _commentaires.fr.md frontmatter, and commentaires.html.twig, including the enable_on_routes entries. Start by checking the template's use of page.header values and the included partials/comments.html.twig; done means the comments page renders without the reported error, although the issue does not specify the intended fix.

Written by the indexing model from the issue text.