Found this KB article on the error I am having. I do not understand the solution. It says to remove the javascript file but that file is embedded in one of the DLLs???
I ended up using JustDecompile to copy the code of the page-editor.js file from the sitefinity frontend dll, and then copied it to my site at the root in Designers/Scripts and that seems to have resolved the issue. I didn't edit the code at all so I am not sure I understand the fix. But at least it is working correctly now.
Had this come up again in a separate website after upgrading form v11.0 to v12.2. After a lot of investigating I realized it was browser specific. In Chrome, I went into the settings and cleared the cache (files and cookies). The AppPath error went away. Must have been a JS file cached from the previous version. Anyway, something easy to try if you are getting this error.