Crash caused by clients that don't support isolated tree mode calling on the secondary thread.
Created attachment 404444 [details] Patch
Committed r264470: <https://trac.webkit.org/changeset/264470> All reviewed patches have been landed. Closing bug and clearing flags on attachment 404444 [details].
<rdar://problem/65675613>
*** Bug 214408 has been marked as a duplicate of this bug. ***
*** Bug 214407 has been marked as a duplicate of this bug. ***