spec fixes

Author: lukemckinstry

packages/engine/Source/Scene/Model/ModelInstanceCollection.js

@@ -49,9 +49,9 @@ import RuntimeError from "../../Core/RuntimeError.js";
 function ModelInstanceCollection(options) {
   this._instances = [];
   this._dirty = false;
-  this._model = options.model ?? undefined;
+  this._model = options?.model ?? undefined;
 
-  this.initialize(options.instances);
+  this.initialize(options?.instances);
 }
 
 Object.defineProperties(ModelInstanceCollection.prototype, {
@@ -105,7 +105,7 @@ ModelInstanceCollection.prototype.add = function (transform) {
   Check.typeOf.object("transform", transform);
   //>>includeEnd('debug');
 
-  if (this._model._loader._hasMeshGpuInstancing) {
+  if (this._model?._loader._hasMeshGpuInstancing) {
     throw new RuntimeError(
       "Models with the EXT_mesh_gpu_instancing extension cannot use the ModelInstanceCollection class.",
     );

packages/engine/Specs/Scene/Model/ModelInstanceSpec.js

@@ -139,17 +139,16 @@ describe(
         fixedFrameTransform,
       );
 
-      const instance = new ModelInstance(instanceModelMatrix);
-
       const model = await loadAndZoomToModelAsync(
         {
           gltf: sampleGltfUrl,
-          instances: [instance],
-          modelMatrix: Matrix4.IDENTITY,
+          instances: [instanceModelMatrix],
         },
         scene,
       );
 
+      const instance = model.instances.get(0);
+
       const boundingSphere = instance.getBoundingSphere(model);
 
       const boundingSphereCenter = new Cartesian3(
@@ -177,12 +176,10 @@ describe(
         fixedFrameTransform,
       );
 
-      const instance = new ModelInstance(instanceModelMatrix);
-
       const model = await loadAndZoomToModelAsync(
         {
           gltf: sampleGltfUrl,
-          instances: [instance],
+          instances: [instanceModelMatrix],
           modelMatrix: new Matrix4.fromTranslation(
             new Cartesian3(
               1253565.1903550967,
@@ -194,6 +191,8 @@ describe(
         scene,
       );
 
+      const instance = model.instances.get(0);
+
       const boundingSphere = instance.getBoundingSphere(model);
 
       const boundingSphereCenter = new Cartesian3(