I have a save where I can reproduce a crash by putting the cube in a ultradense furnace. Once it finishes the recipe it crashes with:

The mod Ultracube: Age of Cube (0.2.12) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event Ultracube::on_tick (ID 0)
LuaEntity API call when LuaEntity was invalid.
stack traceback:
[C]: in function 'index'
__Ultracube/scripts/cube_search.lua:319: in function 'cube_search_inventories'
Ultracube/scripts/cube_search.lua:516: in function 'cube_search_local'
Ultracube/scripts/cube_search.lua:867: in function 'update'
Ultracube/scripts/cube_fx.lua:323: in function 'tick'
Ultracube/control.lua:269: in function <Ultracube/control.lua:268>

Do you want the save, or is the stack trace enough?

Yes, the save would be very useful thanks

Ok, I narrowed it down a little. It crashes when it tries to move it with an inserter. If I remove the inserter the recipe finishes without an issue.
Tried various types of chests and configurations but I can't tell why this one crashes and not other ones.

https://drive.google.com/file/d/1GPMIjT9Fsm_juQ6EtovwcHGIMjx36m8l/view?usp=sharing
There is a furnace right next to the player. Putting the cube in it triggers the crash.

As described on the mod page, Ultracube needs to know when entities that could hold a cube get messed with in order to work properly. You're using a mod called Merging Chests which creates and deletes chests without setting raise_built / raise_destroy, so my code doesn't know about it. Unfortunately unless that gets changed in that mod, I can't do anything other than mark it as incompatible.

Thanks for debugging it!

I posted on the Merging Chests page to let the author know: https://mods.factorio.com/mod/WideChests/discussion/65992abc727fe023bcb11155
Maybe they'll consider fixing it.