Fix editMenuMenuParent breaking when menu has no parent

@robloach, I have confirmed that this issue exists, but this change doesn't seem to fix it. In my testing editMenuMenuParent was always defined, but an error happens later around line 193 where we try to get the parent menu title:

let currentParentTitle = currentOption.innerText.substring(
  currentOption.innerText.indexOf(' '),
);

currentOption has just been defined as null. so getting innerText is not possible. I think this is a pretty big problem of a bug, but I don't have the time to try and debug it at the moment.