Fix InstancedStaticMesh RemoveInstance invalidating indices

When removing instances from an ISMC, indices / indexes returned from AddInstance will be invalidated. This can cause issues for users that want to do custom management of instances. Using something like a TMap keyed off of uint32 where the ID is just a running counter could solve this issue.

1. Create a new BP only project.
2. Create an actor with an Instanced Static Mesh Component.
3. Assign some mesh to the ISMC.
4. Recreate the BeginPlay events in the attached image.
5. Add the Actor to a world and PIE.

Expected: The transform will be retrieved successfully
Actual: The transform cannot be retrieved.