The single-layer path removed the layer with no guard, leaving a document
with no layers and no active layer. It now refuses with the same error as
the multi-select path; the edit is discarded, so nothing changes and no
history step is recorded.
Co-authored-by: prekabreki <prekabreki@users.noreply.github.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
