The error happens when clicking "show requestors train stops and priority assignments" > "e". Detailed log can be found below:

Error while running event train-pubsub::on_gui_click (ID 1)
train-pubsub/control.lua:1299: attempt to index field '?' (a nil value)
stack traceback:
train-pubsub/control.lua:1299: in function <train-pubsub/control.lua:924>
206.193 Error ServerMultiplayerManager.cpp:92: MultiplayerManager failed: "The mod Train Supply Manager (1.1.18) caused a non-recoverable error.
Please report this error to the mod author.

I was browsing the code.

local backer_name, cur_pub = resource_id(element.name, 8)
...
gui_open(player, global.newpublishers[player.surface.name][backer_name][cur_pub].entity, true)

A bit more context, I had multiple renames/edit of both source ids and its supplystations. To fix the crash I introduced a new resource id pair, added the supply stations and updated all requester station to use the new pair, and deleted the old one. Now this works and I cannot reproduce it anymore...

OK. Here is the save that can produce it: https://drive.google.com/file/d/1ZDeERDXwTTtsdXNEiW9bympg0_IFOEqE/view?usp=sharing
Open the request station list, click the "e" on any requester which uses virtual-signal as the 2nd resource id will crash the game.

I use (item, signal-E) pair for the empty train of that item and I use signal-Any to for universal dumping station

I can add some code protection .. looks like you deleted the requester lights.