get method
Reads the document referenced by this DocumentReference.
If no document exists, the read will return null.
Implementation
Future<DocumentSnapshot?> get() async {
  debug('REQUEST: GET $documentUri\n');
  http.Response response =
      await httpr.get(Uri.encodeFull(documentUri), headers: headers);
  int httpStatusCode = response.statusCode;
  debug('RESPONSE: $httpStatusCode\n${response.body}');
  if (httpStatusCode == HttpStatus.ok) {
    Map<String, dynamic> jsonResponse =
        json.decode(response.body) as Map<String, dynamic>;
    _id = jsonResponse['id'] as int;
    return DocumentSnapshot._(path, jsonResponse);
  } else {
    return null;
  }
}