I think this is an uneeded script. Although I still haven't been able to produce the symptom locally, I comment out the script registration and it still works as expected.
You can proably solve it on your end by clearing the contents from the /ClientScript/MyPageMenu.js file so that it is just an empty text file.
Please give that a try and let me know. Might be good to back it up first in case that doesn't work.
Thanks,
Joe