Error while running event toolbars-mod::on_nth_tick(5)
LuaGuiElement API call when LuaGuiElement was invalid.
stack traceback:
[C]: in function 'newindex'
...t/sections/section/content/table/slots/item/ItemSlot.lua:236: in function 'refreshCount'
...t/sections/section/content/table/slots/item/ItemSlot.lua:93: in function 'handle'
__toolbars-mod/src/player/EventBus.lua:17: in function 'publish'
toolbars-mod/src/player/Inventory.lua:39: in function 'refresh'
toolbars-mod/src/player/Player.lua:168: in function 'refreshInventory'
toolbars-mod/control.lua:180: in function
<toolbars-mod/control.lua:174>

Please describe circumstances. What had happened before? Had you hidden toolbars etc.?

Clicked on another mod's menu button in the upper-left toolbar (vanilla) on the screen, and it threw that error.
Can repo it with simply loading in and clicking on the button leading to the other mod's primary menu.
Mod menu in question added by: https://mods.factorio.com/mod/RPG
Can't repo it in 2.8.1...

However, making a new mod toolbar and fiddling with the toggle and lock quick-bar buttons lead to this crash;
The mod Toolbars (2.8.1) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event toolbars-mod::on_lua_shortcut (ID 38)
LuaGuiElement API call when LuaGuiElement was invalid.
stack traceback:
[C]: in function 'newindex'
__toolbars-mod/src/gui/Component.lua:245: in function 'hide'
toolbars-mod/src/gui/toolbar/Toolbar.lua:197: in function 'update'
toolbars-mod/src/gui/toolbar/Toolbar.lua:188: in function 'handle'
toolbars-mod/src/player/EventBus.lua:17: in function 'publish'
toolbars-mod/src/player/Player.lua:163: in function 'handle'
toolbars-mod/src/player/Player.lua:112: in function 'toggleToolbars'
toolbars-mod/control.lua:41: in function <toolbars-mod/control.lua:35>

What mods do you have installed?

Base
Elevated Rails
Quality
AAI Loaders
AAI Loaders: Electric powered
AAI Signal Transmission
AAI Vehicles: Chaingunner
AAI Vehicles: Flame Tank
AAI Vehicles: Flame Tumbler
AAI Vehicles: Hauler
AAI Vehicles: Ironclad
AAI Vehicles: Laser Tank
AAI Vehicles: Miner
AAI Vehicles: Warden
Aircraft (Updated for Space Age)
Aircraft Realism
Armoured Biters
Assembly Analyst
Autostash
Belt Visualizer
BeltRouter Lite
Better Alert Arrows
Bottleneck Lite
Bullet Trails
Buoyant Spidertrons
Burner upcycler
Change recipe quality without re-selecting recipe
Circuit Connection Visualizer
Circuit HUD V2
Color Coded Pipe Planner
Color Coded Pipes
Cybersyn Combinator
Disco Science
Electric Trains
Enable all Feature Flags
Enemy Expansion Notifications
Equipment Gantry
Even Distribution
Factorio Library
Factory Finder
Factory Planner
Filter HHelper
Floaty Chat
Floor Placer
Flow Configuration
Fluid level indicator
Fluid Must Flow
Glowing Laser Beams
Grappling Gun
GUI Unifyer: Unified
Hazard Stripes
HelicopterRecolored
HelicopterRevival
Hidden Loot
Immersive locomotives
InformaTron
Jetpack
King Jo's Item Bunker
King Jo's Vehicle Horns
Kryzeth Standard Library
Kuxynator's Core Library
Kuxynator's Smart Linked Chests
Lex's Aircraft
Mammoth-MK3
Manual Inventory Sorting
MFerrari lib
Milestones
Mineable Wreckage
Mining Drones 2 Remastered
Mining Patch Planner
Modular Turrets
NiceFill
P.U.M.P.
Packing Tape
Picklocks Inserter block detacher
Project Cybersyn
Pyanodons Alien Life
Pyanodons Alien Life Graphics 1
Pyanodons Alien Life Graphics 2
Pyanodons Alien Life Graphics 3
Pyanodons Alternative Energy
Pyanodons Alternative Energy Graphics
Pyanodons Coal Processing
Pyanodons Coal Processing Graphics
Pyanodons Fusion Energy
Pyanodons Fusion Energy Graphics
Pyanodons High Tech
Pyanodons High Tech Graphics
Pyanodons Industry
Pyanodons Petroleum Handling
Pyanodons Petroleum Handling Graphics
Pyanodons Post Processing
Pyanodons Raw Ores
Pyanodons Raw Ores Graphics
Pyanodons Simplified
Rail Logistics Dispatcher 2
RailTester
Random Colors
Realistic Ores
Realistic Reactor Glow
Relocatable Shipwreck
Robot Attrition
Rocket-Silo Construction
Rotatable Equipment
RPG
SA-Humvee
SA-T1 Atomic Tank
Schall Ammo Turrets
Schall Arachnid Platoon
Schall Armoured Train
Schall Armoured Train: Nostalgia
Schall Belt Configuration
Schall Chunk Alignment
Schall Circuit Group
Schall Combat Robotics
Schall Geowarfare
Schall Gun Pod
Schall Inserters
Schall Language Pack
Schall Map Tag
Schall Minor Tweaks
Schall Missile Command
Schall Oil Fuel
Schall Pickup Tower
Schall Radar Alignment
Schall Recharging Weapon
Schall Remains & Pickup
Schall Satellite Controller
Schall Tank Platoon
Schall Transport Group
Schall Uncraft
Sensible Deconstruction
Shield Projector
Show my damage
Simple Adjustable Inserters
Spider Bots
Spidertron Weapon Loadout
Staged Blueprint Planning
Stats GUI
Task List
Toolbars
Uncrafting Logistic Chests
Upcycler
Valves
Vehicle Corpses
VehicleSnap
Water-Friendly Walls
Wire Tools
YARM - Resource Monitor