Fixed occasional NullReferenceExceptions coming from ObjectManipulator.OnPointerDown() (#10613)

Author: marek-stoj

Assets/MRTK/SDK/Features/Input/Handlers/ObjectManipulator.cs

@@ -577,6 +577,8 @@ public Vector3 GetPointerGrabPoint(uint pointerId)
         public virtual void OnPointerDown(MixedRealityPointerEventData eventData)
         {
             if (eventData.used ||
+                    eventData.Pointer == null ||
+                    eventData.Pointer.Result == null ||
                     (!allowFarManipulation && eventData.Pointer as IMixedRealityNearPointer == null))
             {
                 return;