Fix nullref (#3460)

Co-authored-by: sebastienlagarde <sebastien@unity3d.com>

Author: adrien-de-tocqueville

com.unity.render-pipelines.high-definition/CHANGELOG.md

@@ -26,6 +26,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Fixed model import by adding additional data if needed.
 - Fix screen being over-exposed when changing very different skies.
 - Fixed incorrect debug wireframe overlay on tessellated geometry (using littessellation), caused by the picking pass using an incorrect camera matrix.
+- Fixed nullref in layered lit shader editor.
 
 ### Changed
 - Changed Window/Render Pipeline/HD Render Pipeline Wizard to Window/Rendering/HDRP Wizard

com.unity.render-pipelines.high-definition/Editor/Material/LayeredLit/LayeredLitGUI.cs

@@ -348,7 +348,7 @@ public static void InitializeMaterialLayers(AssetImporter materialImporter, ref
                         layers[i] = AssetDatabase.LoadAssetAtPath(AssetDatabase.GUIDToAssetPath(layersGUID.GUIDArray[i]), typeof(Material)) as Material;
                     }
                 }
-                if (layersGUID.withUV.Length > 0)
+                if (layersGUID.withUV != null && layersGUID.withUV.Length > 0)
                 {
                     withUV = new bool[layersGUID.withUV.Length];
                     for (int i = 0; i < layersGUID.withUV.Length; ++i)