Description of problem:
When starting xfce4-display-settings, it crashes with SIGSEGV.

Version-Release number of selected component (if applicable):
xfce4-settings-4.12.1-1.el7.x86_64

How reproducible:
The problem sometimes happen but I have not identified the reproducible step.


Actual results:
After starting xfce4-display-settings, a window appears but disappears soon within less than 1 second.

Expected results:
xfce4-display-settings should show a dialog and keep it open until user close it.


Additional info:
GDB shows as below.
$ gdb xfce4-display-settings
(gdb) run
Starting program: /usr/bin/xfce4-display-settings
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
Detaching after fork from child process 8341.

(xfce4-display-settings:8337): xfce4-display-settings-CRITICAL **: 21:18:51.786: xfce_randr_mode_width: assertion 'mode != NULL' failed

(xfce4-display-settings:8337): xfce4-display-settings-CRITICAL **: 21:18:51.786: xfce_randr_mode_height: assertion 'mode != NULL' failed

Program received signal SIGSEGV, Segmentation fault.
convert_xfce_output_info (output_id=output_id@entry=0) at main.c:1626
1626            output->rate = mode->rate;
(gdb) bt
#0  0x0000000000407aec in convert_xfce_output_info (output_id=output_id@entry=0) at main.c:1626
#1  0x00000000004080c0 in dialog_map_event_cb (window=<optimized out>) at main.c:2679
#2  0x00000000004080c0 in dialog_map_event_cb (app=0x85e0f0) at main.c:2716
#3  0x00000000004080c0 in dialog_map_event_cb (widget=<optimized out>, event=<optimized out>, data=0x85e0f0) at main.c:2730
#4  0x00007ffff7479bdc in _gtk_marshal_BOOLEAN__BOXED () at /lib64/libgtk-x11-2.0.so.0
#5  0x00007ffff6496a68 in g_closure_invoke () at /lib64/libgobject-2.0.so.0
#6  0x00007ffff64a93d9 in signal_emit_unlocked_R () at /lib64/libgobject-2.0.so.0
#7  0x00007ffff64b0dbc in g_signal_emit_valist () at /lib64/libgobject-2.0.so.0
#8  0x00007ffff64b13bf in g_signal_emit () at /lib64/libgobject-2.0.so.0
#9  0x00007ffff75a9dc4 in gtk_widget_event_internal () at /lib64/libgtk-x11-2.0.so.0
#10 0x00007ffff747839b in gtk_main_do_event () at /lib64/libgtk-x11-2.0.so.0
#11 0x00007ffff6ec392c in gdk_event_dispatch () at /lib64/libgdk-x11-2.0.so.0
#12 0x00007ffff5972099 in g_main_context_dispatch () at /lib64/libglib-2.0.so.0
#13 0x00007ffff59723f8 in g_main_context_iterate.isra.19 () at /lib64/libglib-2.0.so.0
#14 0x00007ffff59726ca in g_main_loop_run () at /lib64/libglib-2.0.so.0
#15 0x00007ffff7477247 in gtk_main () at /lib64/libgtk-x11-2.0.so.0
#16 0x000000000040cd17 in display_settings_show_main_dialog (display=<optimized out>) at main.c:2824
#17 0x0000000000407054 in main (argc=1, argv=0x7fffffffe4a8) at main.c:3126
(gdb) p mode
$1 = (const XfceRRMode *) 0x0
(gdb) l
1621        if (output->on)
1622        {
1623            output->rotation = xfce_randr->rotation[output_id];
1624            output->width = mode->width;
1625            output->height = mode->height;
1626            output->rate = mode->rate;
1627        }
1628        else
1629        {
1630            output->rotation = 0;