fix null reference when adding new layers in a map patch

1 files changed, 9 insertions, 2 deletions

diff --git a/src/SMAPI/Framework/Content/AssetDataForMap.cs b/src/SMAPI/Framework/Content/AssetDataForMap.cs
index 8674461e..b8722ead 100644
--- a/src/SMAPI/Framework/Content/AssetDataForMap.cs
+++ b/src/SMAPI/Framework/Content/AssetDataForMap.cs
@@ -99,8 +99,15 @@ namespace StardewModdingAPI.Framework.Content
             }
 
             // get target layers
-            IDictionary<Layer, Layer> sourceToTargetLayers = source.Layers.ToDictionary(p => p, p => target.GetLayer(p.Id));
-            HashSet<Layer> orphanedTargetLayers = new HashSet<Layer>(target.Layers.Except(sourceToTargetLayers.Values));
+            Dictionary<Layer, Layer> sourceToTargetLayers =
+                (
+                    from sourceLayer in source.Layers
+                    let targetLayer = target.GetLayer(sourceLayer.Id)
+                    where targetLayer != null
+                    select (sourceLayer, targetLayer)
+                )
+                .ToDictionary(p => p.sourceLayer, p => p.targetLayer);
+            HashSet<Layer> orphanedTargetLayers = new(target.Layers.Except(sourceToTargetLayers.Values));
 
             // apply tiles
             bool replaceAll = patchMode == PatchMapMode.Replace;