Affects me too. When I remove laptop from its dock and the DisplayPort monitor is disconnected, xfsettingsd crashes. Full log:

$ XFSETTINGSD_DEBUG=1 xfsettingsd --no-daemon
xfce4-settings(xsettings): _XSETTINGS_S0 registered on screen 0
xfce4-settings(xsettings): 30 settings changed (serial=0, len=1232)
xfce4-settings(xsettings): resource manager (xft) changed (len=1330)
xfce4-settings(displays): Detected CRTC 63.
xfce4-settings(displays): Detected CRTC 64.
xfce4-settings(displays): Detected CRTC 65.
xfce4-settings(displays): Detected CRTC 66.
xfce4-settings(displays): Detected output 67 eDP1.
xfce4-settings(displays): Detected output 71 DP2.
xfce4-settings(displays): CRTC 63 assigned to eDP1.
xfce4-settings(displays): CRTC 63, output list[0] -> 67.
xfce4-settings(displays): CRTC 64 assigned to DP2.
xfce4-settings(displays): DP2 will be disabled by configuration.
xfce4-settings(displays): Total 1 active output(s).
xfce4-settings(displays): Normalized CRTC 63: size=1920x1080, pos=0x0.
xfce4-settings(displays): CRTC 63 must be temporarily disabled.
xfce4-settings(displays): Disabling CRTC 63.
xfce4-settings(displays): min_h = 8, min_w = 8, max_h = 32767, max_w = 32767, prev_h = 2160, prev_w = 1920, prev_hmm = 572, prev_wmm = 508, h = 1080, w = 1920, hmm = 286, wmm = 508.
xfce4-settings(displays): Applying desktop dimensions: 1920x1080 (px), 508x286 (mm).
xfce4-settings(displays): Configuring CRTC 63.
xfce4-settings(displays): Applying changes to CRTC 63.
xfce4-settings(displays): Configuring CRTC 64.
xfce4-settings(displays): Applying changes to CRTC 64.
xfce4-settings(displays): Disabling CRTC 64.
xfce4-settings(displays): Configuring CRTC 65.
xfce4-settings(displays): Configuring CRTC 66.
xfce4-settings(pointers): initialized xi 2.3
xfce4-settings(pointers): [SynPS/2 Synaptics TouchPad] Changed device property Synaptics_Edge_Scrolling
xfce4-settings(pointers): [SynPS/2 Synaptics TouchPad] Changed device property Synaptics_Tap_Action
xfce4-settings(pointers): [SynPS/2 Synaptics TouchPad] Changed device property Device_Enabled
xfce4-settings(pointers): [SynPS/2 Synaptics TouchPad] Changed device property Synaptics_Two-Finger_Scrolling
xfce4-settings(pointers): [SynPS/2 Synaptics TouchPad] Changed device property Synaptics_Circular_Scrolling
xfce4-settings(pointers): [SynPS/2 Synaptics TouchPad] Changed device property Synaptics_Circular_Scrolling_Trigger
xfce4-settings(pointers): [TPPS/2 IBM TrackPoint] Changed device property Device_Enabled
xfce4-settings(keyboards): initialized xkb 1.0
xfce4-settings(keyboards): set auto repeat on
xfce4-settings(keyboards): set key repeat (delay=350, rate=33)
xfce4-settings(keyboards): set numlock off
xfce4-settings(accessibility): AccessXKeys disabled
xfce4-settings(accessibility): stickykeys disabled
xfce4-settings(accessibility): slowkeys disabled
xfce4-settings(accessibility): bouncekeys disabled
xfce4-settings(accessibility): mousekeys disabled
xfce4-settings(keyboard-shortcuts): 28 shortcuts loaded
xfce4-settings(displays): RRScreenChangeNotify event received.
xfce4-settings(displays): Refreshing RandR cache.
xfce4-settings(displays): Detected CRTC 63.
xfce4-settings(displays): Detected CRTC 64.
xfce4-settings(displays): Detected CRTC 65.
xfce4-settings(displays): Detected CRTC 66.
xfce4-settings(displays): Detected output 67 eDP1.
xfce4-settings(displays): Detected output 71 DP2.
xfce4-settings(displays): Noutput: before = 2, after = 2.
xfce4-settings(displays): RRScreenChangeNotify event received.
xfce4-settings(displays): Refreshing RandR cache.
xfce4-settings(displays): Detected CRTC 63.
xfce4-settings(displays): Detected CRTC 64.
xfce4-settings(displays): Detected CRTC 65.
xfce4-settings(displays): Detected CRTC 66.
xfce4-settings(displays): Detected output 67 eDP1.
xfce4-settings(displays): Detected output 71 DP2.
xfce4-settings(displays): Noutput: before = 2, after = 2.
xfce4-settings(displays): RRScreenChangeNotify event received.
xfce4-settings(displays): Refreshing RandR cache.
xfce4-settings(displays): Detected CRTC 63.
xfce4-settings(displays): Detected CRTC 64.
xfce4-settings(displays): Detected CRTC 65.
xfce4-settings(displays): Detected CRTC 66.
xfce4-settings(displays): Detected output 67 eDP1.
xfce4-settings(displays): Detected output 71 DP2.
xfce4-settings(displays): Noutput: before = 2, after = 2.
xfce4-settings(displays): RRScreenChangeNotify event received.
xfce4-settings(displays): Refreshing RandR cache.
xfce4-settings(displays): Detected CRTC 63.
xfce4-settings(displays): Detected CRTC 64.
xfce4-settings(displays): Detected CRTC 65.
xfce4-settings(displays): Detected CRTC 66.
xfce4-settings(displays): Detected output 67 eDP1.
xfce4-settings(displays): Detected output 71 DP2.
xfce4-settings(displays): Noutput: before = 2, after = 2.
xfce4-settings(fontconfig): monitoring 112 paths
xfce4-settings(workspaces): 1 desktop names set from xfconf
xfce4-settings(displays): RRScreenChangeNotify event received.
xfce4-settings(displays): Refreshing RandR cache.
xfce4-settings(displays): Detected CRTC 63.
xfce4-settings(displays): Detected CRTC 64.
xfce4-settings(displays): Detected CRTC 65.
xfce4-settings(displays): Detected CRTC 66.
xfce4-settings(displays): Detected output 67 eDP1.
xfce4-settings(displays): Noutput: before = 2, after = 1.
xfce4-settings(displays): Output disconnected: DP2
xfce4-settings(displays): Disabling CRTC 89.
The program 'xfsettingsd' received an X Window System error.
This probably reflects a bug in the program.
The error was 'BadRRCrtc (invalid Crtc parameter)'.
  (Details: serial 1152 error_code 148 request_code 140 minor_code 21)
  (Note to programmers: normally, X errors are reported asynchronously;
   that is, you will receive the error a while after causing it.
   To debug your program, run it with the --sync command line
   option to change this behavior. You can then get a meaningful
   backtrace from your debugger if you break on the gdk_x_error() function.)