Fix build

src/Materials/PBR/pbrMaterial.ts

@@ -196,7 +196,7 @@ export class PBRMaterial extends PBRBaseMaterial {
      */
     @serializeAsTexture()
     @expandToProperty("_markAllSubMeshesAsTexturesDirty")
-    public metallicReflectanceTexture: BaseTexture;
+    public metallicReflectanceTexture: Nullable<BaseTexture>;
 
     /**
      * Used to enable roughness/glossiness fetch from a separate channel depending on the current mode.