Crash data indicates that we are attempting to process a form where the document's frame has been nulled out. We don't have a reproducible test case to trigger this behavior, so the following patch attempts to trap the unusual condition that seems likely to be the cause of the crash.
Created attachment 335961 [details]
Comment on attachment 335961 [details]
View in context: https://bugs.webkit.org/attachment.cgi?id=335961&action=review
r=me, except I'm not a WebKit2 reviewer.
> + // Beartrap for <rdar://problem/37579354>
Uber-Nit. WebKit style says comments should end with a period.
Committed r229683: <https://trac.webkit.org/changeset/229683>