Comment 9 for bug 1206314

Line 1520 in that file looks like so:

    g_error ("The mapping function given to g_settings_get_mapped() for key "
             "'%s' in schema '%s' returned FALSE when given a NULL value.",
             key, g_settings_schema_get_id (settings->priv->schema));

which is a pretty good indicator of what's probably happening here.

Quoth the docs:

If the mapping function fails for all possible values, one additional attempt is made: the mapping function is called with a NULL value. If the mapping function still indicates failure at this point then the application will be aborted.