Following fixes the behaviour, but I'm not sure if that's the proper fix.
Thanks for looking into this, and thanks Caefix for reporting.
Digging through the code a bit, I believe the fix is correct, so I checked it in. All the other callbacks used with libfselect->file_select are set up to accept NULL.
Note this bug did not apply to 1.4, because the 5227 commit was never merged. I merged both.