```
The mod Modular Chests (2.0.12) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event LB-Modular-Chests::on_built_entity (ID 6)
Invalid ItemID: expected LuaItemPrototype, LuaItemStack, LuaItem or string.
stack traceback:
[C]: in function 'insert'
LB-Modular-Chests/control.lua:150: in function 'SpawnChest'
LB-Modular-Chests/control.lua:175: in function 'PlaceChest'
LB-Modular-Chests/control.lua:340: in function 'CheckTotalLengthAll'
LB-Modular-Chests/control.lua:353: in function 'HandlePlacedEntity'
LB-Modular-Chests/control.lua:2: in function <LB-Modular-Chests/control.lua:1>
``

Seems to happen when building a modular chest while arms are inserting into it.

Thanks for the report, I am able to reproduce the error.
I'll try to get it fixed asap.

Error occurred when trying to merge chests with contents in their inventories
The cause was that the get_contents() function changed its return type from dictionary[string → uint] to array[ItemCountWithQuality]

Pushing an update to the mod that fixes the issue now.