The mod Sam's Trains (ALPHA) (0.18.3) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event SamTrain_v18::on_nth_tick(120)
LuaGuiElement API call when LuaGuiElement was invalid.
stack traceback:
SamTrain_v18/gui/overview.lua:220: in function 'refresh'
SamTrain_v18/scripts/conductor.lua:309: in function 'tick'
SamTrain_v18/scripts/conductor.lua:653: in function <SamTrain_v18/scripts/conductor.lua:652>
stack traceback:
[C]: in function 'index'
__SamTrain_v18/gui/overview.lua:220: in function 'refresh'
SamTrain_v18/scripts/conductor.lua:309: in function 'tick'
SamTrain_v18/scripts/conductor.lua:653: in function <SamTrain_v18/scripts/conductor.lua:652>

what other mods are you using? There may be an incompatibility.

So, i'm having the same problem:

The mod Sam's Trains (0.18.4) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event SamTrain_v18::on_nth_tick(120)
LuaGuiElement API call when LuaGuiElement was invalid.
stack traceback:
SamTrain_v18/gui/overview.lua:220: in function 'refresh'
SamTrain_v18/scripts/conductor.lua:309: in function 'tick'
SamTrain_v18/scripts/conductor.lua:653: in function <SamTrain_v18/scripts/conductor.lua:652>
stack traceback:
[C]: in function 'index'
__SamTrain_v18/gui/overview.lua:220: in function 'refresh'
SamTrain_v18/scripts/conductor.lua:309: in function 'tick'
SamTrain_v18/scripts/conductor.lua:653: in function <SamTrain_v18/scripts/conductor.lua:652>

build: 0.18.36
os: mac
mod list:
Aircraft
Alien Biomes
Auto Deconstruct
Bottleneck
Factorio Standard Library
Helmod
Informatron
Krastorio 2
Miniloader
Nanobots
Sam's Train
Squeak Through
Waterfill

Thanks for the list of mods... It seems that Informatron is the culprit. I'll see if I can figure out what the issue is.

Thanks man, but I just had to reinstall Sam's Train and the problem was solved. =]