Wait, this was different:
(gdb) bt full
#0 x_error (dpy=0x7ffff7e56460, error=0x7fffffffd610) at demo-Gtk.c:4253
#1 0x00007ffff6d03734 in _XError () at /lib/x86_64-linux-gnu/libX11.so.6
#2 0x00007ffff6d00107 in () at /lib/x86_64-linux-gnu/libX11.so.6
#3 0x00007ffff6d001a5 in () at /lib/x86_64-linux-gnu/libX11.so.6
#4 0x00007ffff6d0145d in _XReply () at /lib/x86_64-linux-gnu/libX11.so.6
#5 0x00007ffff6cfcae1 in XSync () at /lib/x86_64-linux-gnu/libX11.so.6
#6 0x00007ffff7f06f88 in gdk_x11_window_get_xid () at
/lib/x86_64-linux-gnu/libgdk-3.so.0
#7 0x000055555556d36f in launch_preview_subproc (s=0x555555749360) at
demo-Gtk.c:3576
win = 0x555555749320
p = 0x555555749400
id = 93824993337600
new_cmd = 0x555555780d10 "\300\276\267UUU"
forked = -1854788039
cmd = 0x55555599eac0 "squirtorus --root"
pr = 0x5555555dd330
window = 0x5555558a4da0
#8 0x000055555556d9eb in update_subproc_timer (data=0x555555749360)
at demo-Gtk.c:3759
s = 0x555555749360
#9 0x00007ffff718ebe8 in () at /lib/x86_64-linux-gnu/libglib-2.0.so.0
#10 0x00007ffff718e04e in g_main_context_dispatch () at
/lib/x86_64-linux-gnu/libglib-2.0.so.0
#11 0x00007ffff718e400 in () at /lib/x86_64-linux-gnu/libglib-2.0.so.0
#12 0x00007ffff718e4a3 in g_main_context_iteration () at
/lib/x86_64-linux-gnu/libglib-2.0.so.0
#13 0x00007ffff7501fe5 in g_application_run () at
/lib/x86_64-linux-gnu/libgio-2.0.so.0
#14 0x00005555555727a7 in main (argc=2, argv=0x7fffffffdcc8) at demo-Gtk.c:5349
s = 0x7fffffffe0db "/xscreensaver-settings"
