Modular list page with template override throws error

Hallo,

when i use @self.modular in a page and want to edit it with the admin interface i get an error like this:

`An exception has been thrown during the rendering of a template ("Template "modular/fluid-container.html.twig" is not defined.") in "@Page:/home/blumberg/public_html/grav/user/pages/01.home" at line 2.`

But the site is viewed as it should.

When i delete the twig code ...

```
{% for p in page.collection %}
{{ p.content }}
{% endfor %}
```

... then it works well.

Reproduce the error in the admin interface using @self.modular and the supplied collection loop, then trace why modular/fluid-container.html.twig is unavailable while the site renders correctly. Compare the admin and site rendering paths; done means the page can be edited without the template exception and the collection loop remains intact.

Written by the indexing model from the issue text.