Check entity can be loaded before creating snippet.

Check that the entity is loaded before creating a snippet. This prevents fatal errors if the index gets out of sync with the site.

We might want to log when that happens?

src/Form/ChatForm.php

@@ -133,12 +133,14 @@ class ChatForm extends FormBase {
         continue;
       }
 
-      $entity = $index->loadItem($match->getExtraData('metadata')->item_id)->getValue();
-      $content = StringHelper::prepareText(trim($match->getExtraData('metadata')->content), [], 1024);
-      $messages[] = [
-        'role' => 'system',
-        'content' => "Source link: {$entity->toLink()->toString()}\nSnippet: {$content}",
-      ];
+      // If the entity can be loaded, prepare a trimmed version
+      if ($entity = $index->loadItem($match->getExtraData('metadata')->item_id)->getValue()) {
+        $content = StringHelper::prepareText(trim($match->getExtraData('metadata')->content), [], 1024);
+        $messages[] = [
+          'role' => 'system',
+          'content' => "Source link: {$entity->toLink()->toString()}\nSnippet: {$content}",
+        ];
+      }
     }
 
     // Send the query to OpenAI.