Lisp exception when building a box with a non-empty closet

[rigdern]

Repro steps

I reproed this with Boxer 3.4.19 2024-06-24

1. Create a data box named x with the value 4
2. On the next line write build [ @x ] (where [ @x ] is a data box with the value @x)
3. Put your cursor in the data box of the build line.
4. Open the data box's closet.
5. Put something into it like the default __________ [ DRAWER ] __________.
6. Close the data box's closet.
7. Execute the build line

Unexpected result: A Common Lisp exception is thrown:

Lisp Error:
  No applicable methods for #<STANDARD-GENERIC-FUNCTION BOXER:GETPROP 80D12C2FE4>
  with args (:NEW :BOXTOP)

Alternatively, this can be reproed with build repro.box.zip. Screenshot: