May be a non-issue. This happened when trying to place a lock on an invisible wall so that it would open with a door. After the initial error happened, it began to happen any time I looked at the invisible wall I tried to lock.
[ULib] addons/nzombies-unlimited-master/lua/nzombies-unlimited/core/entities_tools/doors_sandbox.lua:160: attempt to concatenate field 'Group' (a nil value)
1. fn - addons/nzombies-unlimited-master/lua/nzombies-unlimited/core/entities_tools/doors_sandbox.lua:160
2. Run - lua/ulib/shared/hook.lua:109
3. determinetargetstr - addons/nzombies-unlimited-master/lua/nzombies-unlimited/core/hudmanagement.lua:315
4. fn - addons/nzombies-unlimited-master/lua/nzombies-unlimited/core/hudmanagement.lua:334
5. Run - lua/ulib/shared/hook.lua:109
6. HUDPaint - gamemodes/base/gamemode/cl_init.lua:75
7. unknown - gamemodes/sandbox/gamemode/cl_init.lua:75
May be a non-issue. This happened when trying to place a lock on an invisible wall so that it would open with a door. After the initial error happened, it began to happen any time I looked at the invisible wall I tried to lock.
https://i.imgur.com/ktwIi0D.png
EDIT: It's worth noting that I actually have functional locks on invisible walls elsewhere in the map, https://i.imgur.com/3PoSqAw.png
I think the issue may be tied to not assigning a door group before trying to put a lock on an invisible wall.