clean install

web/modules/entity_reference_integrity/entity_reference_integrity.module

@@ -0,0 +1,20 @@
+<?php
+
+/**
+ * @file
+ * Module file.
+ */
+
+use Drupal\entity_reference_integrity\EntityReferenceIntegrityEntityHandler;
+
+/**
+ * Implements hook_entity_type_alter().
+ */
+function entity_reference_integrity_entity_type_alter(array &$entity_types) {
+  /** @var \Drupal\Core\Entity\EntityTypeInterface $entity_type */
+  foreach ($entity_types as $entity_type) {
+    if (!$entity_type->hasHandlerClass('entity_reference_integrity')) {
+      $entity_type->setHandlerClass('entity_reference_integrity', EntityReferenceIntegrityEntityHandler::class);
+    }
+  }
+}