Created attachment 8213 
Don't crash if screen is NULL

xfce4-session-4.13.1 crashes on a display with multiple independent X11 screens. The cause is that in xfsm_fadeout_new()  there is a loop over all X11 screens and for each screen gdk_display_get_screen() is called. This function is deprecated and returns NULL for each screen != 0 on gtk 3.24.1. This causes a crash in my setup and is quite annoying since I cannot choose between the logout options. I attach a quick'n'dirty fix.