I can not solve this in a pretty way but i found that if e new subscribedUsersFile is created serializeJson(users, subscribedUsersFile) writes "null" in the file because getSubscribedUsers() returns null becaus the file is empty yet.

if (usersDoc.isNull())
      {
        users = usersDoc.to<JsonObject>();
      }
      else 
      { 
        users = usersDoc.as<JsonObject>();
      }

This was my workaround for the problem.

wirewound, you saved my days trying to figure out the issue why I always get "null" for "/showallusers".

I added your suggestion in bool addSubscribedUser(String chat_id, String from_name) after serializeJson(users, subscribedUsersFile); and the sketch works like a charm.

Thank you @witnessmenow Brian Lough and @wirewound

I can not solve this in a pretty way but i found that if e new subscribedUsersFile is created serializeJson(users, subscribedUsersFile) writes "null" in the file because getSubscribedUsers() returns null becaus the file is empty yet.

if (usersDoc.isNull())
      {
        users = usersDoc.to<JsonObject>();
      }
      else 
      { 
        users = usersDoc.as<JsonObject>();
      }

This was my workaround for the problem.

Unfortunately for unforeseen reasons the suggestion of @wirewound doesn't work always.
Based on his suggestions adding the below by replacing between //users.getOrCreateMember(chat_id, from_name); and subscribedUsersFile.close(); will solve the issue:-

if ((usersDoc.isNull()) && (subscribedUsersFile))
{
subscribedUsersFile.print("{"" + chat_id + "":" + """ + from_name + ""}" );
}
else
{
serializeJson(users, subscribedUsersFile);
users = usersDoc.as();
}