When I add it to the boiler,the game crashed.
Error while running event Ultracube::on_tick (ID 0)
Entity is not crafting-machine.
stack traceback:
[C]: in function 'get_recipe'
Ultracube/scripts/cube_fx.lua:425: in function 'is_cube_working'
Ultracube/scripts/cube_fx.lua:476: in function 'track_victory_statistics'
Ultracube/scripts/cube_fx.lua:497: in function 'tick'
Ultracube/control.lua:276: in function <Ultracube/control.lua:275>

I am getting this as well when putting the cube in a boiler.
Game is unplayable due to this, can't generate any steam.
The only mod that I am using is this one and it's required dependencies. No other mods or suggested optional mods.

The mod Ultracube: Age of Cube (0.3.5) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event Ultracube::on_tick (ID 0)
Entity is not crafting-machine.
stack traceback:
[C]: in function 'get_recipe'
Ultracube/scripts/cube_fx.lua:425: in function 'is_cube_working'
Ultracube/scripts/cube_fx.lua:476: in function 'track_victory_statistics'
Ultracube/scripts/cube_fx.lua:497: in function 'tick'
Ultracube/control.lua:276: in function <Ultracube/control.lua:275>

I'm seeing the same error. I don't know what causes it, but I can share my save file: https://drive.google.com/file/d/1yDR9USbapLo2Ge73Ng2VQNR4xTXUsAJR/view?usp=sharing

The first time it happened in a few minutes. The second time it took about 10 minutes before things crashed.

I've got these mods enabled:

base
aai-loaders
armor-spill-prevention
AutoDeconstruct
automatic-station-painter
Automatic_Train_Painter
better-victory-screen
BottleneckLite
cybersyn
cybersyn-combinator
extended-descriptions
factoryplanner
FactorySearch
flib
Flow Control
FluidWagonColorMask
Inventory Sensor
Krastorio2Assets
Milestones
nixie-tubes
PickerAtheneum
PickerDollies
PickerExtended
PipeVisualizer
pushbutton
RecipeBook
romulinhosresearchqueuealwaystrue
safefill
show-max-underground-distance
simhelper
stack-combinator
stdlib
textplates
trainfactory
Ultracube
UtilizationMonitorBlargh
WireShortcuts

Thanks all, it was an edge-case regression introduced in 0.3.5. Should be fixed in 0.3.6