fix indexing collections in elasticsearch

CHANGELOG.md

@@ -67,6 +67,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 - Fix some sequential and scripted playout build failures when using playlists or marathons
 - Fix erasing playout items and history so all related data is also erased
   - This includes rerun history, unscheduled gaps, build status
+- Fix indexing collections when using Elasticsearch backend
 
 ## [26.1.1] - 2026-01-08
 ### Fixed

ErsatzTV.Infrastructure/Search/ElasticSearchIndex.cs

@@ -1011,6 +1011,8 @@ public class ElasticSearchIndex : ISearchIndex
 
     private static void AddCollections(ElasticSearchItem doc, IEnumerable<Collection> collections)
     {
+        doc.Collection ??= [];
+
         foreach (Collection collection in collections)
         {
             doc.Collection.Add(collection.Name);