message: mol-state: fix null obj handling in update
diff: @@ -311,11 +311,12 @@ function findUpdateRoots(cells: Map<Transform.Ref, StateObjectCell>, tree: State
  function findUpdateRootsVisitor(n: Transform, _: any, s: { roots: Ref[], cells: Map<Ref, StateObjectCell> }) {
  const cell = s.cells.get(n.ref);
  - if (cell && cell.obj === StateObject.Null) return false;
  if (!cell || cell.version !== n.version || cell.status === 'error') {
  s.roots.push(n.ref);
  return false;
  }
  + // nothing below a Null object can be an update root
  + if (cell && cell.obj === StateObject.Null) return false;
  return true;
  }
  