the line is related to the construction of pipes if you deconstruct a miner using fluid.
the error happens if you try to deconstruct a drill that has no "last_user"
can you explain how you build the offending drill(s)?
It is simple to prevent this error with an additional check, but it would help to understand the root cause first
I use the mod Outpost Planner for 1.1 (https://mods.factorio.com/mod/OutpostPlanner1-1) to generate the layout of miners, because I'm lazy. However, that just places ghost entities for you. Then either my personal roboport bots or construction bots from the base's logistic grid actually place the real entities. If the real entities inherit their last user from the ghost entities and if Outpost Planner doesn't properly assign anyone to that then I could see that being a problem.
I'm also playing this in multiplayer with 1 other player who also places miners this way.