The mod Train Construction Site Fork - Factorio 2.1 Update (0.5.4) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event trainConstructionSiteFork::on_built_entity (ID 6)
trainConstructionSiteFork/src/trainassembly.lua:110: attempt to call field 'convertRGBA' (a nil value)
stack traceback:
trainConstructionSiteFork/src/trainassembly.lua:110: in function 'saveNewStructure'
trainConstructionSiteFork/src/trainassembly.lua:1227: in function 'onBuildEntity'
trainConstructionSiteFork/control.lua:97: in function <trainConstructionSiteFork/control.lua:91>

Hi lord_farquat,

Thanks for reporting this! The issue was a missing utility function (convertRGBA) in the new FactoryLib dependency. This has been fixed.

Please update both mods to the latest versions:

FactoryLib → 0.2.2
trainConstructionSiteFork → 0.5.5
You can download them from the Mod Portal. The fix is already uploaded.

Let me know if you run into any other issues!