getObjectType(objectId) {
    if (objectId === '_root') return 'map'
    const object = this.getObject(objectId)
    if (object instanceof Text) return 'text'
    if (object instanceof Table) return 'table'
    if (Array.isArray(object)) return 'list'
    return 'map'
  }
Returns a string that is either 'map', 'table', 'list', or 'text', indicating
the type of the object with ID `objectId`.