The mod Train construction site (0.2.19) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event trainConstructionSite::on_built_entity (ID 6)
The mod Smarter Trains (5.1.1) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event SmartTrains::on_entity_renamed (ID 59)
SmartTrains/control.lua:1175: attempt to index field '?' (a nil value)
stack traceback:
SmartTrains/control.lua:1175: in function 'decreaseStationCount'
SmartTrains/control.lua:1230: in function <SmartTrains/control.lua:1226>
stack traceback:
[C]: in function 'newindex'
__trainConstructionSite/src/traincontroller.lua:851: in function 'onBuildEntity'
trainConstructionSite/control.lua:67: in function <trainConstructionSite/control.lua:60>

Also posted on the Smarter Trains mod page.

Setup: I made a closed oval rail loop, placed all the trainbuilder elements and upon placing the 'Trainbuilder Controller' the crash will occur. The closed track does not contain any other train stops besides 1x 'Traindepot'.
Link to screenshot: https://imgur.com/a/o4woGu2