> There are three issues that I see with your commmit:
>
> Issue-1:  face-at-point broken?
> ===============================
>
> M-x toggle-debug-on-error RET
> M-x find-function RET face-at-point RET
> C-x w h
> C-x w r
>
>     Debugger entered--Lisp error: (error "Not a face: nil")
>       signal(error ("Not a face: nil"))
>       error("Not a face: %s" nil)
>       check-face(nil)
>       face-name(nil)
>       hi-lock--regexps-at-point()
>       byte-code("\203\305C\207\306 \203.	<\203.\n\203.\307\310\215\207\204!\311\312!\210\313 \314\f\204-\315\2022\316\317\f@\"\320\305\320\211\f&)C\207" [current-prefix-arg last-nonmenu-event use-dialog-box hi-lock-interactive-patterns defaults t display-popup-menus-p snafu (byte-code "\301\302\303\304\305\306\"BB\"\206.\307\310\311\"\207" [hi-lock-interactive-patterns x-popup-menu t keymap "Select Pattern to Unhighlight" mapcar #[(pattern) "@\301\302@\303A@A@A@!#\304\211B@F\207" [pattern format "%s (%s)" symbol-name nil] 6] throw snafu ("")] 7) error "No highlighting to remove" hi-lock--regexps-at-point completing-read "Regexp to unhighlight: " format "Regexp to unhighlight (default %s): " nil] 8)
>       call-interactively(unhighlight-regexp nil nil)
>
> The reason is faceprop happens to be a string
>
> (get-char-property (point) 'face) 
>  : "hi-yellow"
>
> Issue-2:  Various issues with unhighlighting 
> ============================================
>
> Once you fix Issue-1 you will run in to other issues with
> un-highlighting.  Try highlighting and UN-highlighting in following 3
> ways
>
> 1. Buffer with font-lock-mode ON
> 2. Buffer with font-lock-mode OFF
> 3. Unhighlight from the menu
>
> Caveat:  Extra testing needed if /type/ of face names are changed 
> =================================================================
>
> hi-lock-face-defautls is currently a list of face names (stringp).  If
> it is made a defcustom, it will be cast to a list of symbols (symbolp).
> In some places, face names are expected and in some other places face as
> a symbol is used.  So you need to re-run the tests if move from
> string->symbols.
>
> Suggestion: In default faces, don't mix bold and foreground/background
> =======================================================================
>
> I am OK with defcustom of faces.  Something like
>
>     (defcustom hi-lock-face-defaults
>       '(hi-yellow hi-pink hi-green hi-blue hi-black-b
>                   hi-blue-b hi-red-b hi-green-b hi-black-hb)
>       "Default faces for hi-lock interactive functions."
>       :type '(repeat face)
>       :group 'hi-lock-faces)
>
> Bonus points if the default settings of the faces that go in there is
> revised as part of this bug.  I want to highlight variables in a buffer.
> So consistent policy of highlighting - a changed background of normal
> face - will require no additional work.
>
> Here is how my own faces look like.  Note that the first 4 come from
> "blue" space and the later 4 or so come from "pink" space, all chosen
> using agave.
>
> ps:  I will let you install a change for the above issues.