Currently using Concrete CMS v8.5.7 in this case.
I’m not entirely sure why this is happening as what I’m trying to do here seems reasonable. I’m trying to test searching behaviour for Express Entries when using an Express List Block. And in this particular case the Express Object, one of the Attributes is User Selector.
When I put down an Express List Block onto a testing page (dev, not prod), and select my Express Object, I select a few Attributes one can search for, which includes the Attribute that is User Selector type. But when I hit save for the Block and the page tries to render this, I get this debug error:
Call to undefined method Concrete\Attribute\UserSelector\Controller::searchForm()
Referring to:
…/blocks/express_entry_list/controller.php239
And absolute path:
/var/www/c5-scratch/updates/concrete5-8.5.7/concrete/blocks/express_entry_list/controller.php
I’m really not seeing why this is happening, and I might be the first person to encounter this issue. Uhhh… what can I do exactly? 
And of course this renders the page unusable so I have to go to the sitemap and revert the version.
I also want to add that in this case I am not applying a custom template to the block (so far as I can tell).
