LegacyCustomProtocolManager is incorrectly using the NetworkProcess object off the main thread. NetworkProcess is only safe to use on the main thread. Found this the hardware when I tried adding a threading assertion in 272402@main.
Pull request: https://github.com/WebKit/WebKit/pull/22175
Committed 272437@main (ca9aa80579f0): <https://commits.webkit.org/272437@main> Reviewed commits have been landed. Closing PR #22175 and removing active labels.
<rdar://problem/120017485>