A modif-tree containing a nil shape id crashed the WASM heap write
with "Cannot read properties of null (reading '__u32_buffer')" in
propagate-modifiers. Production builds elide the upstream asserts,
so a nil id (e.g. update-dimensions called with a missing shape's
parent id in the variants flow) reached the uuid heap write.
Drop nil-keyed modif-tree entries in apply-wasm-modifiers and
set-wasm-modifiers before they reach propagate-modifiers and
set-structure-modifiers, filter nil ids at the parse level, and
guard the variant reposition/creation events against missing shapes.
AI-assisted-by: hy3
94a6ba4afb