It's an Emacs 25.1 bug, solved in 25.2
This is an instance of Emacs Bug#26310 which can be resolved by updating to Emacs 25.2, the relevant patch is here.
*ERROR*: Wrong type argument: stringp, nil is from
emacsclient, if you see this it means that git did succeed in calling it, so things are probably fine from git's point of view (i.e., it accepted the given options, and did find the executable).
The code that emacsclient executes runs inside a
condition-case so you won't see any backtrace from errors even if you've set
debug-on-error. You can get a backtrace by setting
*Backtrace* buffer will show up in the main Emacs frame, see #24616.17). An example backtrace that this bug would produce is here, the interesting part is at the top:
Debugger entered--Lisp error: (wrong-type-argument stringp nil)
(if (and (boundp (quote isearch-mode)) isearch-mode) (progn (isearch-cancel)))
(save-current-buffer (set-buffer buffer) (if (and (boundp (quote isearch-mode)) isearch-mode) (progn (isearch-cancel))))
error: There was a problem with the editor '"c:/Users/George/Desktop/Emacs25/bin/emacsclient.exe"'.
Please supply the message using either -m or -F option.
This is from git, and (apart from the editor path) is just a generic error message that it prints when something goes wrong; especially, the suggestion about using
-F should be ignored.