The takeSnapshot callback in -[_WKThumbnailView _requestSnapshotIfNeeded] assumes that taking the snapshot will always succeed. If it fails and the callback receives a null ShareableBitmap::Handle, as can happen if the WKView is closed while a snapshot request is pending, the callback will dereference null.
<rdar://problem/16704660>
Created attachment 230702 [details] Patch
Committed r168195: <http://trac.webkit.org/changeset/168195>