Shortcut theming triggers warnings

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.

1. You changed the theme hook to dashboards_admin_shortcuts, which doesn't exist.
2. Wouldn't it make more sense to either fix the custom theming or to get rid of it altogether? The current solution just leaves a bunch of dead code that will never be executed.

DieterHolvoet → changed the visibility of the branch 2.1.x to hidden.

Here's another idea: why not add our own Shortcuts dashboard block instead of altering the core one, without using a lazy builder? I started a new MR.