This is with the latest git versions of xi-editor and xi-gtk.
$ gdb ./xi-gtk --g-fatal-warnings
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Reading symbols from ./xi-gtk...done.
Starting program: xi-gtk/build/xi-gtk --g-fatal-warnings
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/usr/lib/libthread_db.so.1".
** (xi-gtk:8761): CRITICAL **: xi_line_set_cursors: assertion 'json_cursors != NULL' failed
Thread 1 "xi-gtk" received signal SIGTRAP, Trace/breakpoint trap.
0x00007ffff61a3011 in ?? () from /usr/lib/libglib-2.0.so.0
(gdb) bt
#0 0x00007ffff61a3011 in () at /usr/lib/libglib-2.0.so.0
#1 0x00007ffff61a441d in g_logv () at /usr/lib/libglib-2.0.so.0
#2 0x00007ffff61a457f in g_log () at /usr/lib/libglib-2.0.so.0
#3 0x000000000040dddd in xi_line_set_cursors (self=, json_cursors=0x0) at xi-gtk/Line.vala:27
#4 0x000000000041098a in xi_lines_cache_update (self=, update=0x76aaa0) at xi-gtk/LinesCache.vala:90
#5 0x000000000040c372 in xi_edit_view_update (self=0x68e430, view_id=0x885900 "view-id-1", update=0x76aaa0) at xi-gtk/EditView.vala:223
#6 0x000000000040a433 in _xi_edit_view_update_xi_core_connection_update_received (_sender=, view_id=0x885900 "view-id-1", update=0x76aaa0, self=0x68e430)
at xi-gtk/EditView.vala:61
#7 0x0000000000408d5a in g_cclosure_user_marshal_VOID__STRING_BOXED (closure=0x734730, return_value=0x0, n_param_values=3, param_values=0x7fffffffd920, invocation_hint=0x7fffffffd8a0, marshal_data=0x0) at xi-gtk/CoreConnection.vala:17
#8 0x00007ffff6475f75 in g_closure_invoke () at /usr/lib/libgobject-2.0.so.0
#9 0x00007ffff6487f82 in () at /usr/lib/libgobject-2.0.so.0
#10 0x00007ffff6490bcc in g_signal_emit_valist () at /usr/lib/libgobject-2.0.so.0
#11 0x00007ffff6490faf in g_signal_emit () at /usr/lib/libgobject-2.0.so.0
#12 0x0000000000407374 in xi_core_connection_receive (self=) at xi-gtk/CoreConnection.vala:62
#13 0x00000000004088de in _xi_core_connection_receive_gpollable_source_func (pollable_stream=0x712f40, self=0x713120) at CoreConnection.c:1263
#14 0x00007ffff619d45a in g_main_context_dispatch () at /usr/lib/libglib-2.0.so.0
#15 0x00007ffff619d810 in () at /usr/lib/libglib-2.0.so.0
#16 0x00007ffff619d8bc in g_main_context_iteration () at /usr/lib/libglib-2.0.so.0
#17 0x00007ffff675752d in g_application_run () at /usr/lib/libgio-2.0.so.0
#18 0x0000000000406a20 in xi_application_main (args=0x7fffffffe148, args_length1=2) at xi-gtk/Application.vala:87
#19 0x0000000000406a69 in main (argc=2, argv=0x7fffffffe148) at xi-gtk/Application.vala:85
(gdb)