EntityReferenceItem is fragile about entity save order

1. +++ b/core/includes/theme.inc
   @@ -1493,6 +1493,7 @@ function template_preprocess_feed_icon(&$variables) {
   +  $variables['attributes']['class'] = array('feed-icon');
   

   is this related?

2. +++ b/core/lib/Drupal/Core/Field/Plugin/Field/FieldType/EntityReferenceItem.php
   @@ -208,6 +212,9 @@ public function preSave() {
   +    if (empty($this->target_id) && $this->entity) {
   +      $this->target_id = $this->entity->id();
   

   this needs code comment

3. +++ b/core/modules/system/templates/feed-icon.html.twig
   @@ -15,4 +15,4 @@
   -<a href="{{ url }}"{{ attributes.addClass('feed-icon') }}>{{ icon }}</a>
   +<a href="{{ url }}"{{ attributes }}>{{ icon }}</a>
   
   +++ b/core/modules/system/templates/status-messages.html.twig
   @@ -24,13 +24,7 @@
   -  <div class="{{ attributes.addClass(classes).class }}" role="contentinfo" aria-label="{{ status_headings[type] }}">
   +  <div class="messages messages--{{ type }}" role="contentinfo" aria-label="{{ status_headings[type] }}">
   

   unrelated?

+++ b/core/lib/Drupal/Core/Field/Plugin/Field/FieldType/EntityReferenceItem.php
@@ -193,6 +194,9 @@ public function isEmpty() {
+    if ($this->entity && $this->entity instanceof Entity && !$this->entity->isNew()) {
+      return FALSE;
+    }
     // Allow auto-create entities.
     if ($this->hasUnsavedEntity()) {
       return FALSE;

@@ -208,6 +212,13 @@ public function preSave() {
       $this->target_id = $this->entity->id();
     }
+    // Handle the case where an unsaved entity was directly set using the public
+    // 'entity' property and then saved before this entity. In this case
+    // ::hasUnsavedEntity() will return FALSE but $this->target_id will still be
+    // empty.
+    if (empty($this->target_id) && $this->entity) {
+      $this->target_id = $this->entity->id();
+    }

It looks like those two cases could be combined better then?

Can't the two checks simply be combined to a single $this->entity instanceof EntityInterface in isEmpty() and below, remove the target_id call inside hasUnsavedEntity() ?

Looks good. You had to be running into this or you wouldn't have created the issue, so I guess others will too.

This was a random fail.

Committed 7340fee and pushed to 8.0.x. Thanks!