Google chat bot Appscript error "TypeError: Cannot read property 'space' of undefined onAddToSpace"

Currently I'm exploring the google chat bot and I tried to test the script that I found in google but I encounter an issue, is there someone that encountered same issue as mine?

I still no an idea how to troubleshoot it since this is my first time creating this can someone help me out with this? Thank you in advance.

Error

TypeError: Cannot read property 'space' of undefined
onAddToSpace    @ Code.gs:27

Sample script

function onMessage(event) {
  var name = "";

  if (event.space.type == "DM") {
    name = "You";
  } else {
    name = event.user.displayName;
  }
  var message = name + " said \"" + event.message.text + "\"";

  return { "text": message };
}

function onAddToSpace(event) {
  var message = "";

  if (event.space.singleUserBotDm) {
    message = "Thank you for adding me to a DM, " + event.user.displayName + "!";
  } else {
    message = "Thank you for adding me to " +
        (event.space.displayName ? event.space.displayName : "this chat");
  }

  if (event.message) {

  message = message + " and you said: \"" + event.message.text + "\"";
  }

  return { "text": message };
}

function onRemoveFromSpace(event) {
  console.info("Bot removed from ",
      (event.space.name ? event.space.name : "this chat"));
}