after migration from 4.11 to 4.12 my AGS java script website return this error when I.m using dojo.byid:
Uncaught (in promise) TypeError: dojo.byId is not a function
It is to hard to tell with seeing your code. It is unlikely that all you did was change the url from 4.11 to 4.12 and now receive this error.
I had the same issue when upgrading. Clearly the domReady event is firing prematurely, before dojo is fully ready. I just changed dojo.byId to window.getElementById to get around that bug.