Thank your for your quick action! It's not quite fixed though... I did however figure out specific circumstances: this only happens when deleting an area containing a Memory Unit from the Memory Storage mod. (https://mods.factorio.com/mod/deep-storage-unit)
I'm assuming this is because that's converted to a different item upon manual deletion and that's not properly triggered by mass deletion. Understanding this makes the error easy to avoid though, so if it's a lot of work to fix or has a big impact on performance, there is no need to support that. In that case you may at most consider adding a little note to your mod description to delete those manually first or avoid areas containing those.
