We believe this occured when someone tried to deconstruct a ship with bots:

Error while running event cargo-ships::on_tick (ID 0)
LuaEntity API call when LuaEntity was invalid.
stack traceback:
cargo-ships/control.lua:217: in function <cargo-ships/control.lua:211>

i was not able to remove the ship with bots, but i guess the error could've occurred when mining the oil-platform with bots. should be fixed now.

it is also possible that the water that part of the ship was in may have been landfilled (by bots or by hand).