(running current version, 0.3.1)
this already happened twice now, even though i haven't even researched armor plating yet.
it sometimes happens when i manually rebuild a destroyed gun turret (vanilla turret), meaning there was a ghost (vanilla turret) entity.

Error while running event vtk-armor-plating::on_built_entity (ID 6)
LuaEntity AP call when Lua Entity was ivalid.
stack traceback:
[]vtk-armor-plating/control.lua:72: in functon 'update_armor_equipment'
[]vtk-armor-plating/control.lua:152: in function <vtk-armor-plating/control.lua:150>

i think you should add "entity_to_armor = nil" in the first line of the function set_entity_to_armor.
gonna try it and reporting back in 1-2 days

Hello,

Thanks for the report. I ran some tests and I couldn't reproduce the issue so far. Let me know if you find a reproductible case and I'll fix it asap.

since i added the fix it didn't crash anymore.
to repoduce the bug, try placing a vehicle and picking it up again. then build a building.

Hello thanks for your report I could reproduce and fix the crash in 0.3.2.