byID<T>  static method 
Implementation
static DataRepository<T>? byID<T>(String? id) {
  if (id == null) return null;
  return instances[id] as DataRepository<T>?;
}static DataRepository<T>? byID<T>(String? id) {
  if (id == null) return null;
  return instances[id] as DataRepository<T>?;
}