Update babylon.babylonFileLoader.ts

Fixing the loader when no multi materials are available.

src/Loading/Plugins/babylon.babylonFileLoader.ts

@@ -1343,7 +1343,7 @@
                     if (parsedMesh.materialId) {
                         var materialFound = (loadedMaterialsIds.indexOf(parsedMesh.materialId) !== -1);
 
-                        if (!materialFound) {
+                        if (!materialFound && parsedData.multiMaterials) {
                             for (var multimatIndex = 0; multimatIndex < parsedData.multiMaterials.length; multimatIndex++) {
                                 var parsedMultiMaterial = parsedData.multiMaterials[multimatIndex];
                                 if (parsedMultiMaterial.id == parsedMesh.materialId) {