fix(room): guard RoomPreviewer.updatePreviewModel against null _planeParser

After dispose() nulls out the internal _planeParser /
_backgroundSprite refs, any further updatePreviewModel call
crashed with 'this._planeParser is null'. React 19 StrictMode
in dev double-mounts effects (setup, cleanup, setup again),
which can briefly leave a consumer holding a stale reference
to a disposed previewer between the two setup runs. Bail
silently in that window instead of crashing the editor.

packages/room/src/RoomPreviewer.ts

@@ -136,6 +136,12 @@ export class RoomPreviewer
 
     public updatePreviewModel(model: string, wallHeight: number, scale: boolean = true): void
     {
+        // Defensive: dispose() nulls _planeParser, and React 19
+        // StrictMode dev double-mount can leave a stale reference
+        // briefly pointing at a disposed instance. Bail rather
+        // than crashing with "cannot read property reset of null".
+        if(!this._planeParser) return;
+
         const parser = new FloorHeightMapMessageParser();
 
         parser.flush();