Thanks for the report.
We will solve this crash.
Unfortunatly, we will display an error message here rather than an empty sequence diagram.
Because in your case, PlantUML thinks that you are writing here a class diagram, something like:
@startuml
hide footbox
class autonumber
scale 0.5
skinparam shadowing false
@enduml
So the program will guess that you have forgotten the "class" keyword before "autonumber". This would transform your example into a valid diagram.
Sorry about that!