it tried to automatically replace a belt with items on it with a loader from Krastorio 2 then this errorr happened every time then i disabled the mod, replaced the loader manually and enabled the mod again and the error no longer shows up whe i go close to that area.

Error while running event autobuild::on_nth_tick(10)
Unknown type: false
stack traceback:
[C]: in function 'create_entity'
autobuild/control.lua:247: in function 'try_upgrade_with_stack'
autobuild/control.lua:285: in function <autobuild/control.lua:279>
(...tail calls...)
autobuild/control.lua:375: in function 'do_autobuild'
autobuild/control.lua:470: in function 'handle_player_update'
autobuild/control.lua:476: in function <autobuild/control.lua:474>

Thanks for the report, I need to look into the situation.
I guess, because the krastorio loader is special kind of thing, which has to be treated differently

This should be fixed now with 0.5.6