Plugin block Derivatives

Am having trouble with creating blocks on the fly via my module.

I found information about Derivatives and Block Plugins. So far I can get the blocks to appear in the “Place block” modal,
but when I click on the “Place block” button in the modal I get an error.

TypeError: Drupal\block\BlockForm::getPluginForm(): Argument #1 ($block) must be of type Drupal\Core\Block\BlockPluginInterface, bool given, called in /Users/cy.jobes/Websites/foobar.org-d9-dev/src/web/core/modules/block/src/BlockForm.php on line 128 in Drupal\block\BlockForm->getPluginForm() (line 436 of /Users/cy.jobes/Websites/foobar.org-d9-dev/src/web/core/modules/block/src/BlockForm.php).

I get this error in the Drupal logs
Warning: Trying to access array offset on value of type null in Drupal\Core\Block\BlockBase->getMachineNameSuggestion() (line 242 of /Users/cy.jobes/Websites/foobar.org-d9-dev/src/web/core/lib/Drupal/Core/Block/BlockPluginTrait.php).

I’ve been following the tutorials I can find, but nothing seems to work. Can anyone tell me what I am doing wrong, or how to fix this?

Below are the two files used:
/Plugin/Block

<?php

namespace Drupal\foobar_gallery\Plugin\Block;

use Drupal\Core\Block\BlockBase;
use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
use Drupal\foobar_gallery\Controller\ImageDataController;
use Symfony\Component\DependencyInjection\ContainerInterface;
/**
 * Provides a generic Menu block.
 *
 * @Block(
 *   id = "foobar_gallery_blocks",
 *   admin_label = @Translation("Gallery Block"),
 *   deriver = "Drupal\foobar_gallery\Plugin\Derivative\FoobarGalleryBlocks"
 * )
 */
class FoobarGalleryBlocks extends BlockBase implements ContainerFactoryPluginInterface
{

  private $gallery_data;
  private $blockMachineName;

  public function __construct($gallery_id, $block_machine_name) {
    // Get the data object for this gallery
    $imageDataController = new ImageDataController();
    $this->gallery_data = $imageDataController->getGalleryData($gallery_id);
    $this->blockMachineName = $block_machine_name;
  }


  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
    return false;
  }



  /**
   * {@inheritdoc}
   */
  public function build() {
    return [
      '#theme' => 'gallery_slick_accessible',
      '#gallery_data' => $this->gallery_data,
    ];
  }


}

/Plugin/Derivative

<?php

namespace Drupal\foobar_gallery\Plugin\Derivative;

use Drupal\Component\Plugin\Derivative\DeriverBase;
use Drupal\Core\Plugin\Discovery\ContainerDeriverInterface;
use Drupal\foobar_gallery\Controller\ImageDataController;
use Symfony\Component\DependencyInjection\ContainerInterface;

class FoobarGalleryBlocks extends DeriverBase implements ContainerDeriverInterface
{

  /**
   * @inheritDoc
   */
  public static function create(ContainerInterface $container, $base_plugin_id)
    {
      return new static(
      );
    }


    /**
     * @inheritDoc
     */
    public function getDerivativeDefinitions($base_plugin_definition)
    {
      // Get the n ames of the blocks that are saved in the database
      $imageDataController = new ImageDataController();
      $blocks = $imageDataController->getBlockNames();

      /*
       * $blocks output
       * array(3) {
          [0]=>
          object(stdClass)#656 (3) {
            ["id"]=>
            string(1) "1"
            ["block_name"]=>
            string(7) "450x200"
            ["block_machine_name"]=>
            string(15) "gallery_450x200"
          }
          [1]=>
          object(stdClass)#657 (3) {
            ["id"]=>
            string(1) "2"
            ["block_name"]=>
            string(7) "350x100"
            ["block_machine_name"]=>
            string(15) "gallery_350x100"
          }
          [2]=>
          object(stdClass)#658 (3) {
            ["id"]=>
            string(1) "3"
            ["block_name"]=>
            string(4) "fooo"
            ["block_machine_name"]=>
            string(4) "fooo"
          }
        }
       * */


      // Create the blocks
      foreach ($blocks as $block) {
        $this->derivatives[$block->block_machine_name] = $base_plugin_definition;
        $this->derivatives[$block->block_machine_name]['admin_label'] = t('Gallery Block: ') . $block->block_name;
      }
      return $this->derivatives;
    }
}