The following error was produces when adding this mod to an existing saved game.

Error while running event BeltUpgrader::on_tick (ID 0)
Error while running event TheFatController::on_robot_built_entity (ID 14)
TheFatController/control.lua:575: attempt to index field 'created_entity' (a nil value)
stack traceback:
BeltUpgrader/control.lua:272: in function 'revive'
BeltUpgrader/control.lua:291: in function 'creep'
BeltUpgrader/control.lua:113: in function 'checkRoboports'
BeltUpgrader/control.lua:491: in function <BeltUpgrader/control.lua:486>

I probably built the event wrong so TheFatController glitched out. I'll take a look.