This log appears when running many layout and api tests. There are legit cases for which registration would not be null (parallel unregisters or clearing registrations).
Created attachment 334824 [details] Patch
Comment on attachment 334824 [details] Patch Clearing flags on attachment: 334824 Committed r229149: <https://trac.webkit.org/changeset/229149>
All reviewed patches have been landed. Closing bug.
<rdar://problem/38042544>