Thanks.  Does the patch below help in any way with these crashes?

If not, it sounds like I'm out of my depth here.  The crashes you show
are deep in the bowels of Glib called by GTK; reading the (annoyingly
minimal) documentation of those APIs, I sense the danger of some basic
issue with calling these APIs from several threads.  If what the GTK
documentation tells everything there is to it, then the patch below
should fix these problems.  But if not, then I guess we will need help
from a GTK expert.