Comment 18 for bug 1665048

l3eta@l3eta-ubuntu:~/Desktop$ gdb xfce4-display-settings
GNU gdb (Ubuntu 7.11-0ubuntu1) 7.11
Copyright (C) 2016 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying"
and "show warranty" for details.
This GDB was configured as "i686-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
<http://www.gnu.org/software/gdb/documentation/>.
For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from xfce4-display-settings...(no debugging symbols found)...done.
(gdb) run
Starting program: /usr/bin/xfce4-display-settings
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/i386-linux-gnu/libthread_db.so.1".
[New Thread 0xb6547b40 (LWP 3475)]
[New Thread 0xb5d46b40 (LWP 3476)]

(xfce4-display-settings:3471): xfce4-display-settings-CRITICAL **: xfce_randr_mode_width: assertion 'mode != NULL' failed

(xfce4-display-settings:3471): xfce4-display-settings-CRITICAL **: xfce_randr_mode_height: assertion 'mode != NULL' failed

Thread 1 "xfce4-display-s" received signal SIGSEGV, Segmentation fault.
0x800060b3 in ?? ()
(gdb) backtrace
#0 0x800060b3 in ?? ()
#1 0x80006688 in ?? ()
#2 0xb7bc4f0b in ?? () from /usr/lib/i386-linux-gnu/libgtk-x11-2.0.so.0
#3 0xb77f0b6b in g_closure_invoke () from /usr/lib/i386-linux-gnu/libgobject-2.0.so.0
#4 0xb78032b9 in ?? () from /usr/lib/i386-linux-gnu/libgobject-2.0.so.0
#5 0xb780b88c in g_signal_emit_valist () from /usr/lib/i386-linux-gnu/libgobject-2.0.so.0
#6 0xb780bfc5 in g_signal_emit () from /usr/lib/i386-linux-gnu/libgobject-2.0.so.0
#7 0xb7ce4fc4 in ?? () from /usr/lib/i386-linux-gnu/libgtk-x11-2.0.so.0
#8 0xb7bc393a in gtk_main_do_event () from /usr/lib/i386-linux-gnu/libgtk-x11-2.0.so.0
#9 0xb7a36fa9 in ?? () from /usr/lib/i386-linux-gnu/libgdk-x11-2.0.so.0
#10 0xb7542e29 in g_main_context_dispatch () from /lib/i386-linux-gnu/libglib-2.0.so.0
#11 0xb75430c9 in ?? () from /lib/i386-linux-gnu/libglib-2.0.so.0
#12 0xb7543479 in g_main_loop_run () from /lib/i386-linux-gnu/libglib-2.0.so.0
#13 0xb7bc2485 in gtk_main () from /usr/lib/i386-linux-gnu/libgtk-x11-2.0.so.0
#14 0x8000b914 in ?? ()
#15 0x80005560 in main ()
(gdb)

My graphics card is a Raedon R5 220 PCI
I have a LED 1080p Full HD ViewSonic Monitor connected via VGA-0
I have a HDTV HDMI SYLVANIA connected via HDMI-0

I just installed a fresh copy of Xubuntu.