Shortcut themeing throws exceptions

I'm seeing the same issue. However, the exception in the module file is a follow-up error from the issue in the event subscriber. So, I've removed that change in the MR as it is not required.

Another change I've added to the MR is, that in Drupal 10.3 the shortcut block is built with the lazy_builder, and that throws another exception if we try to add a theme to it. I've disabled the modification for now, if there is a lazy_builder in place as I don't know how we could add a theme to that.

There is a related issue in ViewFormSubscriber for the lazy_builder limitation. I've added that to the MR as well.

Had similar issue caused by ViewFormSubscriber when lazy_builder used:
AssertionError: When a #lazy_builder callback is specified, no properties can exist; all properties must be generated by the #lazy_builder callback. You specified the following properties: #post_render. in assert() (line 334 of /var/www/html/drupal/core/lib/Drupal/Core/Render/Renderer.php)

MR https://git.drupalcode.org/project/dashboards/-/merge_requests/26 fixes this and all works fine.