When you upgrade via the regular Aras upgrade service, this one is an Aras problem, not a YOU problem :).
I right now would expect that the error happens in the Method "VE_GetCopyTreeGridHeader" itself. When you come from I12 and upgrade to I31, it´s probably a typical issue caused by obsolete functions. Typical something .NET Core related, but in your case the error message doesn´t match so I can´t tell.
document.thisItem.apply is nothing unusual. Instead of "get","edit",etc. you do something custom. Very often the custom code might do something very similar like regular edits and gets, but does something additional.
When I look at my own code for something comparable I found one Method that use this exact technique for replacement of the regular "promote" button. The Method does some validation, opens a custom promote Form, than does the original "promote" action and a few additional relationship edits and returns the same item that would have been returned when doing a regular promote.
