reverted:
--- b/core/lib/Drupal/Core/Config/Entity/Exception/ConfigEntityIdLengthException.php
+++ /dev/null
@@ -1,15 +0,0 @@
-<?php
-
-/**
- * @file
- * Contains \Drupal\Core\Config\Entity\Exception\ConfigEntityIdLengthException.
- */
-
-namespace Drupal\Core\Config\Entity\Exception;
-
-use Drupal\Core\Config\ConfigException;
-
-/**
- * Exception thrown when a configuration entity ID is too long.
- */
-class ConfigEntityIdLengthException extends ConfigException {}
diff -u b/core/modules/block/custom_block/lib/Drupal/custom_block/Entity/CustomBlock.php b/core/modules/block/custom_block/lib/Drupal/custom_block/Entity/CustomBlock.php
--- b/core/modules/block/custom_block/lib/Drupal/custom_block/Entity/CustomBlock.php
+++ b/core/modules/block/custom_block/lib/Drupal/custom_block/Entity/CustomBlock.php
@@ -182,8 +182,7 @@
     $fields['type'] = FieldDefinition::create('entity_reference')
       ->setLabel(t('Block type'))
       ->setDescription(t('The block type.'))
-      ->setSetting('target_type', 'custom_block_type')
-      ->setSetting('max_length', EntityTypeInterface::BUNDLE_MAX_LENGTH);
+      ->setSetting('target_type', 'custom_block_type');
 
     $fields['log'] = FieldDefinition::create('string')
       ->setLabel(t('Revision log message'))
@@ -191,7 +190,8 @@
     $fields['type'] = FieldDefinition::create('entity_reference')
       ->setLabel(t('Block type'))
       ->setDescription(t('The block type.'))
-      ->setSetting('target_type', 'custom_block_type');
+      ->setSetting('target_type', 'custom_block_type')
+      ->setSetting('max_length', EntityTypeInterface::BUNDLE_MAX_LENGTH);
 
     $fields['log'] = FieldDefinition::create('string')
       ->setLabel(t('Revision log message'))