Custom entity reference field widget for inventory size basis product field

ProductInventorySizeBasisWidget.php

<?php

namespace Drupal\wbc_custom\Plugin\Field\FieldWidget;

use Drupal\Core\Field\FieldDefinitionInterface;
use Drupal\Core\Field\FieldItemListInterface;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Field\Plugin\Field\FieldWidget\EntityReferenceAutocompleteWidget;

/**
 * Plugin implementation of the 'product_inventory_size_basis' widget.
 *
 * @FieldWidget(
 *   id = "product_inventory_size_basis",
 *   label = @Translation("Inventory size basis"),
 *   description = @Translation("An autocomplete text field."),
 *   field_types = {
 *     "entity_reference"
 *   }
 * )
 */
class ProductInventorySizeBasisWidget extends EntityReferenceAutocompleteWidget {

  /**
   * {@inheritdoc}
   */
  public static function isApplicable(FieldDefinitionInterface $field_definition) {
    $entity_type = $field_definition->getTargetEntityTypeId();
    $field_name = $field_definition->getName();
    return $entity_type == 'commerce_product' && $field_name == 'field_inventory_size_basis';
  }

  /**
   * {@inheritdoc}
   */
  public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
    $element = parent::formElement($items, $delta, $element, $form, $form_state);

    if (!$items[$delta]->getEntity()->isNew()) {
      $element['target_id']['#disabled'] = TRUE;
    }

    return $element;
  }

}