This can be triggered when you lose the SVG space invaders game in the linked URL. It's a clean NULL deref, and I'll have a patch shortly.
Created attachment 66807 [details] Patch
Comment on attachment 66807 [details] Patch Good catch, r=me.
Comment on attachment 66807 [details] Patch Clearing flags on attachment: 66807 Committed r66973: <http://trac.webkit.org/changeset/66973>
All reviewed patches have been landed. Closing bug.