file_finder: Fix crash in new_path_prompt (#16991)

Closes #16919 

repro step:
  - add two worktree
  - modify settings `"use_system_path_prompts" : false`
  - `ctrl-n` create new file, typing any chars.
  - `ctrl-s` save file
  - typing any char, crashed.

Release Notes:

- Fixed crashed when setting `"use_system_path_prompts": false` or in
remote project with two or more worktree.