read default value from thirdpartysettings

fieldblock.module

@@ -20,9 +20,8 @@ function fieldblock_form_field_ui_display_overview_form_alter(&$form, &$form_sta
   $bundle = $form['#bundle'];
   $mode = $form['#mode'];
 
-  $config_name = 'fieldblock.' . $entity_type .'.'. $bundle;
-  $config = \Drupal::config($config_name);
-  $settings = $config->get($mode);
+  /** @var \Drupal\Core\Entity\EntityDisplayModeInterface $view_mode_config */
+  $view_mode_config = EntityViewDisplay::load($entity_type . '.' . $bundle . '.' . $mode);
 
   // Add a column header.
   $form['fields']['#header'][] = t('Display as block');
@@ -32,7 +31,7 @@ function fieldblock_form_field_ui_display_overview_form_alter(&$form, &$form_sta
   foreach ($field_names as $field_name) {
     $form['fields'][$field_name]['fieldblock'] = array(
       '#type' => 'checkbox',
-      '#default_value' => isset($settings[$field_name]) ? true : false,
+      '#default_value' => $view_mode_config->getThirdPartySetting('fieldblock', $field_name) ? true : false,
       '#title' => '',
     );
   }