Not all entities have a _self href use UUID instead in dependency graph

+++ b/src/DefaultContentManager.php
@@ -172,36 +172,37 @@ class DefaultContentManager implements DefaultContentManagerInterface {
           // Throw an exception when this URL already exists.
-          if (isset($file_map[$self])) {
+          if (isset($file_map[$item_uuid])) {

this UUID

+++ b/src/DefaultContentManager.php
@@ -172,36 +172,37 @@ class DefaultContentManager implements DefaultContentManagerInterface {
-              '@href' => $self,
-              '@first' => $file_map[$self]->uri,
+              '@uuid' => $item_uuid,
+              '@first' => $file_map[$item_uuid]->uri,
               '@second' => $file->uri,
             );
             // Reset link domain.
             $this->linkManager->setLinkDomain(FALSE);
-            throw new \Exception(SafeMarkup::format('Default content with href @href exists twice: @first @second', $args));
+            throw new \Exception(SafeMarkup::format('Default content with uuid @uuid exists twice: @first @second', $args));

What is uri here exactly? What does it show for paragraph that doesn't have a uri/link?