bug#67658: filesets can’t open files from menu-bar when using ‘:tree’

[. shynur]

Reproduce:

  PS D:/Tmp> dir 
  test.txt test.org
  PS D:/Tmp> runemacs.exe -Q
  
  (require 'filesets)
  (push `("Teest"
          . ((:tree ,default-directory ""))) filesets-data)
  (filesets-init)
  
  M-x eval-buffer 
  
  Click: Menu-Bar -> File -> Filesets -> 0 Teest -> Tree: d:/Tmp/
  
Then emacs just gave me an error:
  “filesets-files-under: Wrong type argument: stringp, nil”
  
I expected it to open all the files under that directory.
Am I missing something?

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v29.1.90 for MS-Windows