`forge--pull-topic` fails with nil topic after creating PR when `selective-p` is true

[jarreds]

Description

When creating a pull request in a repository that has selective-p set to t, the post-submit callback fails with:

error in process filter: Fetching an individual topic not implemented for forge-github-repository

Steps to reproduce

1. Add a repository to forge with selective-p set to t (e.g., via forge-add-repository with "individual topics" option, or programmatically)
2. Create a pull request with forge-create-pullreq
3. Submit the PR

Root cause

In forge--post-submit-callback (forge-post.el:266-289), the topic variable is captured at closure creation time:

(topic (ignore-errors (forge-get-topic forge--buffer-post-object)))

For new PRs, forge--buffer-post-object is the repository, so topic becomes nil.

When the callback executes after successful PR creation:

(if (or (not full-pull)
        (oref repo selective-p))
    (forge--pull-topic repo topic)  ; topic is nil here
  (forge--pull repo))

If selective-p is t, it calls (forge--pull-topic repo nil). There's no method specialization for a nil topic on forge-github-repository, so it falls back to the generic method which signals an error.

Environment

Magit v4.5.0-32-g64fae64, Transient 0.12.0, Forge 0.6.3, Git 2.52.0, Emacs 30.2, darwin

[tarsius]

Thanks for the report!