getWebViewUrl method
Implementation
String? getWebViewUrl(CustomMessage? messageObject) {
  if (messageObject != null &&
      messageObject.customData != null &&
      messageObject.customData!.containsKey("document")) {
    Map? document = messageObject.customData?["document"];
    if (document != null && document.containsKey("document_url")) {
      return document["document_url"];
    }
  }
  return null;
}