Hello! Apologies for the issue, but thanks for the report!
Have been able to reproduce the error locally.
Did a little digging/testing to confirm, and found that it is indeed an error on the side of Virtual Train Manager.
As they list a link to their source code on the mod-portal page for VTM, I would open a pull-request with proposed changes/fixes to allow for compatibility, but alas, the repo would appear to be private.
Essentially, either this line can be added at the very beginning of the on_trainstop_built function:
if not event or not event.entity or not event.entity.valid then return end
or, I noticed the registered event is not making use of an explicit filter; registering the event with this filter:
{{ filter = "name", name = "train-stop", }}
also fixed the issue.
Either of the two above should resolve the error and allow both More Enemies and Virtual Train Manager to be used in tandem.
Thanks again for the report - do let me know if you have any further questions!