Prestashop 8.1.3 upgrade small problem in theme back office

[texxo]

Hello,

i have an old theme that i managed to keep alive since i think version 1.7 by always modifying things according to the errors i see when i upgrade to some newer prestashop version (and PHP).

So, now i downloaded my live website+theme locally and set up PHP 8.1.26, i did the upgrade of the store (which is 1.7.8.7 and PHP 7.4) through the 1 click upgrade.

All went ok, of course it started to be giving me various errors, mostly for the jsencode and decode, ok, i went to each page of my theme and changed to the new thing, all ok.

BUT my theme (which unfortunately is not supported any more by developer for prestashop since they do only magenta versions now) has 1 module in the back office for CMS Blocks which loads all ok but the window where you are editing the HTML text that you want to appear although is in a tinyMCE window, does not render as HTML and it shows all the HTML tags.
This is 1 problem, which ok, it is the least problem.

Second problem and more serious is the module of the theme called mega menu pro, which although when i go to configure it loads normally, it does not show the lower part which actually is where you can structure the menus etc. (i attach an image here).

Meaning, that part i somehow not loaded (i have inspected the page and the texts that should exist, simply are not there, so i guess are not loaded and i suspect something is wrong on how the theme is trying to load the data from the database through jsquery most probably)

In debug mode, nothing pops up, in monitoring console mode from the browser i see jQuery.Deferred exception: CdzMediabrowserUtility is not defined

this CdzMediabrowserUtility though i search and it seems to be normally defined in one of the theme files.

So, my question is:

PHP 8.1.x has incompatibilities with lets say older jscript that was coming with older prestashop versions?

(what i mean at least is this: if it changed the way how things should be called with jscript in order all to be going good under php8.1.x)

if anyone has any similar experience and has something to share, please do, because these 2 things are the only things that do not work right now (all rest in back office is working, front office is all ok, no problem at all - had some strange thing with the block reassurance that was trying to load the images from an unknown position even if it already had loaded the images so i simply kept only the requests from the correct position)

thank you.