Prevent access to "Configure block" page for non-existance themes

There is a page available to add block to any theme, at admin/structure/block/add/page_title_block. Because of that _access_theme: 'TRUE' requirements is avoided from block.admin_add route. Even if we reach the page like you said (admin/structure/block/add/page_title_block/non_existance_theme), we won't be able to create any block as there wont be any options available in the required "Region" field.

Thanks @LiamMorland to report the issue.

Few updates from end so far:

1. I was able to replicate the issue according to the description
2. Did comparison between block.admin_library which defines path admin/structure/block/library/non_existance_theme and block.admin_add which defines path admin/structure/block/add/page_title_block/non_existance_theme have a delta of _access_theme in block.routing.yml file
3. Did some initial debugging and after applying _access_theme under block.admin_add issue is resolved

Keeping the status to "Needs work" since phpunit test is failing, adding "Needs Test" tag and updating the version to "11.x-dev" since i also replicated the same in that version

Keeping the status to "Needs work" since phpunit test is failing

@abhaypai, your patch will cause an "Access denied" to /admin/structure/block/add/page_title_block . In order to avoid that _access_theme: 'TRUE' is avoided from block.admin_add route. We need to add a block even if there is no theme present in the url

Thanks for looking into my suggestion and updating your comments on #2, although i have few points to share:

1. From where did you find the source of this following statement, Any reference link issue is gonna be helpful here to proceed with solution
   

   In order to avoid that _access_theme: 'TRUE' is avoided from block.admin_add route.

2. Can you please share replicating steps to land on this url /admin/structure/block/add/page_title_blockfrom user interface ? I aware of landing on this url directly from url path is possible and forms field shows default theme inputs.
3. Also i cannot see any code use case of this said /admin/structure/block/add/{placeholder} path or block.admin_add route source in the system without being usage of theme name to any pluginid

Looking for suggestions here from other community members; what can be solution of this bug ? Ideally from my POV form should not be visible if there is no theme available or installed in the system.

I ran into this in functional testing with a test that directly requests the page. I don't know of any place where a link to the invalid page exists. It would have been easier to debug that the test was missing the theme if that invalid page had returned a 403 or 404 status.