Hitting backspace too many times causes Lisp exception during search for a named box: Negative subscript -1

[rigdern]

Repro steps

I reproed this with Boxer 3.4.18 2024-05-26

1. Start a search (e.g. by choosing the "Edit > Find" menu item)
2. Start a search for a named box by typing the pipe character, |
3. Hit backspace

Unexpected result: A Common Lisp exception is thrown: Lisp Error:Negative subscript -1, for the first dimension of the array #(NIL #\. NIL NIL NIL NIL NIL NIL)

Expected result: Find switches back to its default search mode (instead of staying in the mode that searches for a named box).