So i tried reproduce it again on a different setup. I placed 3 sets of 2 fences with different lengths, hooked those up to a switch to activate them one by one. See the following overview:
Generator > switch > first fence post
end fences
With this i tried to lure some zombies in and to turn on the switch one at a time. The very first time I tried this, one line gave the error again as seen on the following screenshot: Null reference exception
Even if the player walks through, the error pops up: Player shocked
So for that matter, it is not related to the blood moon event.
After this i restarted my game, and everything worked just fine. I haven't figured out yet what the condition is that triggers this error, restarted a few times now and placed some more lines but so far all works as intended. I will keep trying the next couple of days, if I find something else I will let you know