Hi Rob,
The error occurs because 'RemoveFrom4' receives 'EntityGeneratorRackGenerator_1' which is a SimEntity instead of an EntityContainer. I notice that your model does not create any EntityContainers, so something is missing. Perhaps, the 'Racks' object should be an EntityContainer instead of a SimEntity.
Also, you need to add SimEntities to an EntityContainer with an AddTo object before you can remove them with a RemoveFrom object. Should you be using the AddTo object instead of Combine?
The next release of JaamSim will include a useful error message in this case instead of an obscure Java error.
Harry