assertion failure: g_propagate_error: assertion `src != NULL' failed

The error is occuring here:

#0 0x00007ffff46df4b5 in *__GI_raise (sig=<value optimised out>)
    at ../nptl/sysdeps/unix/sysv/linux/raise.c:64
        pid = <value optimised out>
        selftid = <value optimised out>
#1 0x00007ffff46e2f50 in *__GI_abort () at abort.c:92
        act = {__sigaction_handler = {
            sa_handler = 0x6094e0 <g_threads_got_initialized>,
            sa_sigaction = 0x6094e0 <g_threads_got_initialized>}, sa_mask = {
            __val = {140737307255496, 6891648, 6948896, 2, 140737351988693,
              81, 140737297608256, 140737294052016, 140737353934008,
              140737300433056, 4294967295, 6329568, 140737299816448,
              140737488346960, 10, 140737488345824}}, sa_flags = 6329568,
          sa_restorer = 0x2c5068}
        sigs = {__val = {32, 0 <repeats 15 times>}}
#2 0x00007ffff4c77dfa in IA__g_logv (log_domain=<value optimised out>,
    log_level=<value optimised out>, format=<value optimised out>,
    args1=0x7fffffffdf50) at /build/buildd/glib2.0-2.22.2/glib/gmessages.c:549
        depth = <value optimised out>
        domain = <value optimised out>
        data = 0x0
        log_func = 0x404790
        domain_fatal_mask = -1
        test_level = 10
        was_recursion = 0
        i = <value optimised out>
#3 0x00007ffff4c77e93 in IA__g_log (
    log_domain=0x40d <Address 0x40d out of bounds>, log_level=1037,
    format=0x6 <Address 0x6 out of bounds>)
    at /build/buildd/glib2.0-2.22.2/glib/gmessages.c:569
        args = {{gp_offset = 24, fp_offset = 48,
            overflow_arg_area = 0x7fffffffe030,
            reg_save_area = 0x7fffffffdf70}}
#4 0x00007fffef23064d in apply_configuration_from_filename (
    manager=<value optimised out>,
    filename=0x6a4180 "/home/chr1s/.config/monitors.xml.backup", timestamp=0,
    error=0x7fffffffe088, no_matching_config_is_an_error=<value optimised out>)
    at gsd-xrandr-manager.c:187
        priv = 0x64c0c0
        my_error = 0x0
        success = <value optimised out>
#5 0x00007fffef230a7b in apply_stored_configuration_at_startup (
    manager=0x64c0a0, error=<value optimised out>) at gsd-xrandr-manager.c:1856
        success = <value optimised out>
        intended_filename = 0x6a41d0 "/home/chr1s/.config/monitors.xml"
        my_error = 0x0
        backup_filename = 0x6a4180 "/home/chr1s/.config/monitors.xml.backup"
#6 gsd_xrandr_manager_start (manager=0x64c0a0, error=<value optimised out>)
    at gsd-xrandr-manager.c:1933
        __PRETTY_FUNCTION__ = "gsd_xrandr_manager_start"
#7 0x00007fffef22dcc7 in impl_activate (plugin=0x69de10)
    at gsd-xrandr-plugin.c:78
        res = <value optimised out>
        error = 0x0
#8 0x0000000000405cdd in gnome_settings_plugin_info_activate ()
No symbol table info available.
#9 0x0000000000404975 in ?? ()
No symbol table info available.
#10 0x00007ffff4c8c53c in IA__g_slist_foreach (list=<value optimised out>,
    func=0x404960, user_data=0x0)
    at /build/buildd/glib2.0-2.22.2/glib/gslist.c:743
        next = 0x68ef20
#11 0x000000000040537c in gnome_settings_manager_start ()
No symbol table info available.
#12 0x0000000000404771 in main ()
No symbol table info available.

This might be a gnome-desktop issue as it seems that gnome_rr_config_apply_from_filename_with_time() returns FALSE but doesn't set the GError

Sure enough, that is exactly what is happening according to the info here in GDB. Reassigning to gnome-desktop

The issue is with 100_load_desired_settings.patch. In gnome_rr_config_apply_from_filename_with_time(), there is this new code section:

if (stored) {
     gboolean result = FALSE;

        /* Check framebuffer size. Do not apply
      * the settings if the fb is not big
      * enough
      */
        if (check_framebuffer_size (stored, screen)) {

         result = gnome_rr_config_apply_with_time (stored, screen, timestamp, error);

         /* if we are using the desired settings and
            the settings were applied successfully */
         if (is_desired && result) {
                /* Try to save the desired settings to the default
                 * settings file
                 */
                error = NULL;
                gnome_rr_config_sanitize (stored);
                gnome_rr_config_save (stored, &error);
         }

     }

     gnome_rr_config_free (stored);

     return result;

check_framebuffer_size (stored, screen) evaluates to FALSE on my setup, which means gnome_rr_config_apply_from_filename_with_time() just returns FALSE without setting the GError, which is wrong (FALSE means that there was an error).