exwm: Add TODO about "markerp" error.

.exwm

@@ -4,6 +4,8 @@
 (let ((win (shell-command "conky 2>/dev/null | dzen2 -p -dock -ta l -fn \"DejaVu Sans Mono\" &")))
   ;; Don't pop the output buffer.
   (with-current-buffer (window-buffer win)
+    ;; TODO: Calling fundmental-mode triggers the following error:
+    ;;   error in process filter: Wrong type argument: markerp, nil [2 times]
     (fundamental-mode)                  ; So that we don't display it in the shell lister.
     (rename-buffer " *Async conky|dzen*"))
   (delete-window win))