I couldn't reproduce the problem in GtkLauncher. Steps to reproduce: Using a debug build of webkitgtk+, use Epiphany to open the attached test case. Click the textarea, and do C-a to select all. Then hit backspace: boom.
Created attachment 30156 [details] test case
Created attachment 30157 [details] backtrace
Ah, I also could not reproduce the problem with a release build.
I can't reproduce this with r44682.
Yeah, this seems to be gone. I'll reopen if I find this again.