Helleu,
So I've been dealing with this same situation for a while now but every time I create JSON, the course builds fine but the Menu Button breaks. A blank page will appear until I reload the page and the menu displays fine.
My colleague was able to discern that the menu opacity was still set to 0 upon selecting the Menu button, and by disabling that, the menu displays fine.
Thing is, I don't understand why this happens to me when I just write JSON and change nothing else. my "_id"s and "_parentId"s are all in tact.
So I post on here to raise a potential issue because I was able to break the Menu button fairly easily just by editing JSON. Hopefully, I can gain a better understanding to my issue.