message: StateObjectTracker, check if cell is undefined
diff: @@ -111,7 +111,7 @@ export class StateObjectTracker<T extends StateObject> {
  const changed = this.cell !== cell || this.version !== version;
  this.cell = cell;
  this.version = version || '';
  - this.data = cell.obj ? cell.obj.data as T : void 0 as any;
  + this.data = cell && cell.obj ? cell.obj.data as T : void 0 as any;
  return changed;
  }
  