Merge pull request #9906 from MaxWang-MS/su_demo_fix

SU Demo script fix

Author: MaxWang-MS

Assets/MRTK/Examples/Experimental/SceneUnderstanding/Scripts/DemoSceneUnderstandingController.cs

@@ -144,8 +144,15 @@ public void OnObservationAdded(MixedRealitySpatialAwarenessEventData<SpatialAwar
         public void OnObservationUpdated(MixedRealitySpatialAwarenessEventData<SpatialAwarenessSceneObject> eventData)
         {
             UpdateData(eventData.Id);
-
-            observedSceneObjects[eventData.SpatialObject.SurfaceType][eventData.Id] = eventData.SpatialObject;
+            
+            if (observedSceneObjects.TryGetValue(eventData.SpatialObject.SurfaceType, out Dictionary<int, SpatialAwarenessSceneObject> sceneObjectDict))
+            {
+                observedSceneObjects[eventData.SpatialObject.SurfaceType][eventData.Id] = eventData.SpatialObject;
+            }
+            else
+            {
+                observedSceneObjects.Add(eventData.SpatialObject.SurfaceType, new Dictionary<int, SpatialAwarenessSceneObject> { { eventData.Id, eventData.SpatialObject } });
+            }
         }
 
         /// <inheritdoc />