`:discard` breaks org-agenda-redo when processing the time grid

I have the following configuration at the top level of my configs (I removed irrelevant agenda commands, so beware of unbalanced parens).

```lisp
(setq org-agenda-custom-commands
'(
;; ...
("x" "Overview/span1"
((agenda ""
((org-agenda-start-day "0d")
(org-agenda-span 1)
(org-super-agenda-groups
`((:discard (:deadline (after ,(org-read-date nil nil "+14d"))))
))))))
;; ...
))

```

This configuration works on the first load; however, when I refresh the page, I get the following stack trace (from `emacs --debug-init`) which crashes my agenda view.

```
Debugger entered--Lisp error: (wrong-type-argument stringp nil)
org-super-agenda--group-deadline((... ... ... ... ... ... ... ... ... ... ... ... ... ... ...) (after "2022-02-17"))
org-super-agenda--group-dispatch-discard((... ... ... ... ... ... ... ... ... ... ... ... ... ... ...) (:deadline ...))
org-super-agenda--group-dispatch((... ... ... ... ... ... ... ... ... ... ... ... ... ... ...) (:discard ...))
org-super-agenda--group-items((... ... ... ... ... ... ... ... ... ... ... ... ... ... ...))
org-super-agenda--filter-finalize-entries(#(" northeastern..." 0 28 ... 28 29 ... 29 47 ... 48 50 ... ...))
org-agenda-finalize-entries((... ... ... ... ... ... ... ... ... ... ... ... ... ... ...) agenda)
org-agenda-list(nil)
funcall-interactively(org-agenda-list nil)
call-interactively(org-agenda-list)
#f(compiled-function () #)()
funcall(#f(compiled-function () #))
(let ((org-agenda-start-day '"0d") (org-agenda-span '1) (org-super-agenda-groups '((:discard (:deadline (after "2022-02-17")))))) (funcall '#f(compiled-function () #)))
org-agenda-run-series("Overview/span1" (((agenda "" ((org-agenda-start-day "0d") (org-agenda-span 1) (org-super-agenda-groups `(...)))))))
org-agenda-redo(nil)
funcall-interactively(org-agenda-redo nil)
command-execute(org-agenda-redo)
```

I am currently using org-super-agenda at the current head of master: fb5e2ef.

I understand that relative date support is in the hands of the user (re: #59) -- as an elisp novice, I could easily be doing something wrong. Any help with this bug or my configuration would be appreciated!