Address code review feedback

loaders/src/glTF/2.0/Extensions/KHR_lights.ts

@@ -45,7 +45,7 @@ module BABYLON.GLTF2.Extensions {
                     throw new Error(context + ": Only ambient lights are allowed on a scene");
                 }
 
-                this._loader._babylonScene.ambientColor = light.color ? Color3.FromArray(light.color) : Color3.White();
+                this._loader._babylonScene.ambientColor = light.color ? Color3.FromArray(light.color) : Color3.Black();
 
                 return promise;
             });

src/Materials/PBR/babylon.pbrBaseMaterial.ts

@@ -686,16 +686,16 @@
                 subMesh._materialDefines = new PBRMaterialDefines();
             }
 
-            const scene = this.getScene();
-            const engine = scene.getEngine();
-
             const defines = <PBRMaterialDefines>subMesh._materialDefines;
             if (!this.checkReadyOnEveryCall && subMesh.effect) {
-                if (defines._renderId === scene.getRenderId()) {
+                if (defines._renderId === this.getScene().getRenderId()) {
                     return true;
                 }
             }
 
+            const scene = this.getScene();
+            const engine = scene.getEngine();
+
             if (defines._areTexturesDirty) {
                 if (scene.texturesEnabled) {
                     if (this._albedoTexture && StandardMaterial.DiffuseTextureEnabled) {
@@ -785,7 +785,7 @@
 
             if (!engine.getCaps().standardDerivatives) {
                 let bufferMesh = null;
-                if (mesh instanceof InstancedMesh) {
+                if (mesh.getClassName() === "InstancedMesh") {
                     bufferMesh = (mesh as InstancedMesh).sourceMesh;
                 }
                 else if (mesh instanceof Mesh) {