Warning: Undefined variable $menu_blocks in include_once() (line 66 of themes/contrib/adaptivetheme/at_core/forms/ext/responsive_menus.php).

Tried to reproduce the issue with module version 2.0.3 with D9 instance, and the issue is not reproducible. The AT tool is working fine and i can access the Settings page of the newly generated theme with no issues. Please specify if any other steps are used to reproduce the issue or if it's a theme specific issue.

In my newly made AT Theme I turned on all extensions in the settings of the Theme.

I think the problem arises with the extension "responsive menu"
If I open this one, I'm not able to change the menu (see screenshot). I Only have a drop-down arrow but the list is empty or does not exist.

This happen son more then one of my recently upgraded D10 sites. On those sites, I had to rebuild the theme because I coould not access the settings at all.

One side however works fine. see The second picture I added of how ik looks like on the other site without an error

So somehow, somewhere something has gone wrong.

I would be very gratefull if somebody can offer me a solution.

OK, I figured this one out myself!

On some of my sites i'm using the project Drupal Mega Menu ( https://www.drupal.org/project/we_megamenu → ).
To avoid double menu's I must activate tis menu and deactivate the standard menu in the Block regions.

I just removed the standard menu and then the error occurs.
Putting the standard block menu back but disabling it so it is not visible removed the error.

So in fact it is a problem of interaction between AT Tool and Drupal Mega Menu.

On D9 however this problem did not occur!

OK, I figured this one out myself!

On some of my sites i'm using the project Drupal Mega Menu ( https://www.drupal.org/project/we_megamenu → ).
To avoid double menu's I must activate tis menu and deactivate the standard menu in the Block regions.

I just removed the standard menu and then the error occurs.
Putting the standard block menu back but disabling it so it is not visible removed the error.

So in fact it is a problem of interaction between AT Tool and Drupal Mega Menu.

On D9 however this problem did not occur!