The mod Toggle Build Layer (0.1.3) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event toggle-build-layer::tbl-toggle (ID 362)
Unknown item name: express-transport-belt_panglia_fast_version
stack traceback:
[C]: in function 'find_item_stack'
toggle-build-layer/control.lua:61: in function <toggle-build-layer/control.lua:38>

I was holding an express belt in my hand, clicking the hotkey swaps it to an underground, then clicking again crashes the game.

Nice bug! The mod added an entity with a different name than the item, so I'll have to find a way to reconcile this.

Thanks for the report.