Undefined array key "type" in Drupal\Core\Theme\ThemeManager->render()

Fixes are made on on 11.x (our main development branch) first, and are then back ported as needed according to our policies. Also, 10.2 is in security mode now.

Hi @dmytro.chornyi,

I verified it both in 10.2.x and 11.x, i am not able to reproduce it at my end, I added following code in sites/development.services.yml

twig.config:
    debug: true
    auto_reload: true

Also checked all the check-boxes on admin/config/development/settings page. Adding screenshot for the reference.

It might me the Theme issue. Can you please share all the details like the theme name etc.

Thanks
Samit K.

Don't know if this is relevant but here's a quick fix that worked for me. I'm on Drupal 10.3.1 though, and I faced this exact warning while customising the default user login page using hook_theme_suggestions_alter() and hook_theme_registry_alter(), but this might help others as well, if you are in similar endeavour.

<?php
$theme_registry['page__custom_page']['type'] = 'base_theme_engine';

Setting the type key for the theme suggestion directly in the theme registry resolves it.

Cheers :)
Malhar