I've attached a small C++ program that can be used to reproduce the issue. I compiled it like so

g++ -g3 main.cc -o test-program

Steps to reproduce:
- emacs -Q
- evaluate the forms:
(require 'gdb-mi)
(setq gdb-many-windows t)

- M-x gdb with argument "gdb -i=mi /path/to/test-program"
- Do "run" in the gdb command prompt
- M-x gdb-io-interrupt

Result: error message in minibuffer "Wrong type argument: stringp, nil" (although the command executes normally).

Furthermore, in this state any of the following will also cause the same message to flash:
- doing M-x gud-break in the source buffer
- toggling a breakpoint in the breakpoint buffer
- probably others

Although again none of the commands fail to execute properly.

Root cause:

In the function gdb-frame-handler the form:

(setq gdb-selected-file (file-local-name (gdb-mi--field frame 'fullname)))

causes the error message if gdb-mi--field returns nil. I don't know if this is an expected return value for that function or not. In any case, file-local-name signals the error in case of nil input. The file-local-name wrapping was introduced semi-recently with commit 019c70e731d0243ad3ea2d4ebb02dd4a343e868f.

Thanks.

----------------------------------------

In GNU Emacs 28.0.60 (build 8, x86_64-pc-linux-gnu, GTK+ Version 3.22.30, cairo version 1.15.12)
 of 2021-11-30 built on san
Repository revision: 30553d889d733613e8e5fd22358980baa7ee148e
Repository branch: emacs-28
Windowing system distributor 'The X.Org Foundation', version 11.0.12011000
System Description: CentOS Linux 8

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