The createChapterCue lambda gets dispatched onto a different thread. When it executes, the client might no longer be valid.
Created attachment 445360 [details] Patch
<rdar://problem/85596002>
Created attachment 445378 [details] Patch
Committed r286327 (244686@main): <https://commits.webkit.org/244686@main> All reviewed patches have been landed. Closing bug and clearing flags on attachment 445378 [details].