Adding a filter function to a shell process

["Louis Höfler" <louis.hoefler@mathematek.de>]

Hello everyone,
I try to read output from a (shell "buffer") call, to a string.
I came up with this function:
(defun eide-shell-read-filter (process string)
  (setq eide-shell-output-temp string)
)

(defun ei-shell-read (procid cmd)
  "This function sends commands to a opened shell and stores it's output 
into a variable"
  (eide-debug-message "ei-shell-read: cmd = %s" cmd)
  (eide-debug-message "ei-shell-read: process-filter = %S" 
(process-filter (get-process "shell")))
  (add-function :after (process-filter (get-process "shell")) 
#'eide-shell-read-filter)
  (process-send-string procid (concat cmd "\n"))
  (remove-function (process-filter (get-process "shell")) 
#'eide-shell-read-filter)
  (eide-debug-message "ei-shell-read: command output = %s" 
eide-shell-output-temp)
  eide-shell-output-temp
)

But the above code produces this error for M-x ei-interactive-shell:
Debugger entered--Lisp error: (void-variable v)
   (process-filter v)
   (lambda nil (process-filter v))()
   advice--add-function(:after ((lambda nil (process-filter v)) lambda 
(gv--val) (set-process-filter v gv--val)) eide-shell-read-filter nil)
   ei-shell-read("*msys2-64*" "export")

You can download the full sourcecode of the project here 
http://scm.mathematek.de/repos.cgi/emacside

Can someone help me with this problem?
Thank you, Louis