Fixes Sentry errors where cursorGroupRef.current and wallPreviewRef.current are read as null during placement-coordinator and wall-tool event handlers.

Sentry impact (live as of 2026-05-27 04:00 UTC nightly triage):

• MONOREPO-EDITOR-BC — Cannot read properties of null (reading 'rotation') — 53,478 events since 2026-05-20
• MONOREPO-EDITOR-BD — same family — 403 events since 2026-05-20
• MONOREPO-EDITOR-BE — Cannot set properties of null (setting 'visible') in wall-tool — handled by the wall/tool.tsx hunk
• MONOREPO-EDITOR-9E — same family

Root cause. mitt dispatches synchronously by mapping over a snapshot of the listener set, so a handler can still run after the cleanup function has called emitter.off(...) if the emit is already in flight. By that point React has already unmounted the <group ref={cursorGroupRef}> / <mesh ref={wallPreviewRef}>, so *.current is null and any .rotation / .visible write throws.

Fix. Defensive if (!ref.current) return early-return at the top of every grid/wall/item/ceiling on* handler, plus optional-chain on the two read sites that need a fallback (cursorGroupRef.current?.rotation.y ?? 0).

Two files, +31 / -6 lines:

• packages/editor/src/components/tools/item/use-placement-coordinator.tsx
• packages/nodes/src/wall/tool.tsx

Rebased onto current main 2026-05-27 04:30 UTC; wall/tool.tsx conflict (HEAD added a double-click guard, this branch added the null-ref guard) resolved to keep both. bun run check clean on the touched files.

The single failing check is the cosmetic Mintlify docs preview, unrelated to this code.

🤖 Authored by Anton (Pascal AI assistant). Diagnosis written up in memory/sentry-triage/2026-05-27.md.