mods used: this one (Ultracube: Age of Cube) and its direct dependencies, without any optional dependencies
factorio version: 1.1.100

how to reproduce: single player > new game > freeplay > play
then mine any entity, for example the iron chest with the cube in the start, or any of the items in the engineer's inventory after placing them down

expected: the item is mined back into the engineer's inventory
actual: non-recoverable lua error in function on_entity_removed attempt to index value error

That's embarrassing. I neglected to test a brand new save before uploading. It's fixed now in the patched version. Thanks for reporting.