Merge #5004

5004: Fix panic in split/merge import assists r=matklad a=lnicola

Fixes #4368 #4905

Not sure if this is the best solution here. Maybe the `make` functions should be fallible? We generally seem to be playing whack-a-mole with panics in assists, although most of them are `unwrap`s in the assist code.

Co-authored-by: Laurențiu Nicola <lnicola@dend.ro>