Fix crash when SyncPlay shuffle mode is set to sorted twice

MediaBrowser.Controller/SyncPlay/Queue/PlayQueueManager.cs

@@ -157,7 +157,10 @@ namespace MediaBrowser.Controller.SyncPlay.Queue
         /// </summary>
         public void RestoreSortedPlaylist()
         {
-            if (PlayingItemIndex != NoPlayingItemIndex)
+            // The shuffled playlist is only populated while the shuffle mode is active, so there is
+            // nothing to map back when the playlist is already sorted. Guarding on its contents keeps
+            // a redundant request for the sorted mode from indexing an empty list.
+            if (PlayingItemIndex != NoPlayingItemIndex && _shuffledPlaylist.Count > 0)
             {
                 var playingItem = _shuffledPlaylist[PlayingItemIndex];
                 PlayingItemIndex = _sortedPlaylist.IndexOf(playingItem);

tests/Jellyfin.Server.Implementations.Tests/SyncPlay/PlayQueueManagerTests.cs

@@ -142,6 +142,36 @@ public class PlayQueueManagerTests
         Assert.Equal(Guid.Empty, queue.GetPlayingItemPlaylistId());
     }
 
+    [Fact]
+    public void SetShuffleMode_SortedWhileAlreadySorted_KeepsPlayingItem()
+    {
+        var queue = CreateQueue(3);
+        queue.SetPlayingItemByIndex(1);
+        var expectedItemId = queue.GetPlayingItemId();
+
+        queue.SetShuffleMode(GroupShuffleMode.Sorted);
+
+        Assert.Equal(GroupShuffleMode.Sorted, queue.ShuffleMode);
+        Assert.Equal(1, queue.PlayingItemIndex);
+        Assert.Equal(expectedItemId, queue.GetPlayingItemId());
+    }
+
+    [Fact]
+    public void SetShuffleMode_SortedTwiceAfterShuffle_KeepsPlayingItem()
+    {
+        var queue = CreateQueue(5);
+        queue.SetPlayingItemByIndex(2);
+        var expectedItemId = queue.GetPlayingItemId();
+
+        queue.SetShuffleMode(GroupShuffleMode.Shuffle);
+        queue.SetShuffleMode(GroupShuffleMode.Sorted);
+        queue.SetShuffleMode(GroupShuffleMode.Sorted);
+
+        Assert.Equal(GroupShuffleMode.Sorted, queue.ShuffleMode);
+        Assert.Equal(5, queue.GetPlaylist().Count);
+        Assert.Equal(expectedItemId, queue.GetPlayingItemId());
+    }
+
     [Fact]
     public void SetPlayingItemByIndex_InBounds_SetsPlayingItem()
     {