blocks are null on admin theme I made a sub-theme of the admin theme "Seven" and called it Seven2
Because I wanted to customize the node-edit-form.twig.html file.
That works.

My next step, I want to add a block I made in a custom module, to that node-edit-form.twig.html.
There are several ways of doing that, and one I chose is preprocess page (or node for that matter), where I want to load my block.

The preprocess functions can either be in Seven2 .theme file or the custommodule.module file.

When I execute:

$block = \Drupal\block\Entity\Block::load('checkmark_block');


The $block is NULL when I do kint($block). Its like it can't find the damned block!! I have spent like 4 hours trying to make this thing work, in different ways, but no matter what, I can't load that block in my preprocess function. I don't know why but I think it's because it's an admin theme? Or... please help.

One of the blocks located in custommodule/src/Plugin/Block:

&lt;?php
namespace Drupal\custommodule\Plugin\Block;
use Drupal\Core\Block\BlockBase;

/**
 * Provides a 'CheckMark' Block.
 *
 * @Block(
 *   id = "checkmark_block",
 *   admin_label = @Translation("CheckMark block"),
 *   category = @Translation("CheckMarks"),
 * )
 */
class CheckMark extends BlockBase {

  /**
   * {@inheritdoc}
   */
  public function build() {
    $markup = 'Test test';

    $build = array(
      '#type' =&gt; 'markup',
      '#markup' =&gt; $markup,
      '#cache' =&gt; array('max-age' =&gt; 0),
    );

    return $build;
  }

}
Ok, I made it work by the answer from this post: 
How can I programmatically display a block?

And rendered it by adding this in hook_form_alter:

if ($form_id == 'node_article_edit_form') {
  ... code from the other post here ... 
  $block = $plugin_block-&gt;build();
  $form['checkmark_block'] = $block;
}


And then in the twig file:

{{ form.checkmark_block }}


I find it way to difficult for a small task like this, and annoying to spend so much time on it, but one hopefully learns by it.