documentFromPath method
Returns a reference to a document using its path.
Implementation
DatabaseReference documentFromPath(String path) {
  if (path.isEmpty) {
    throw NullIDException("Cannot get document reference from an empty path");
  }
  if (!path.contains("/")) {
    throw NullIDException("Cannot get document reference from invalid path: $path");
  }
  return RDB.instance.ref(path);
}