Reproduce:
* gnucash --nofile
* File > Import > Import QIF...
* Forward, Select file, Forward, Forward
* Deselect entry by Ctrl-clicking a few times on it
* Back, Forward
->
In unknown file:
   ?: 0* [qif-import:fix-from-acct #f "abc"]
In $prefix/share/gnucash/scm/qif-import/qif-dialog-utils.scm:
 558: 1* [for-each #<procedure #f (xtn)> ...
 562: 2*  [qif-file:xtns #f]
In unknown file:
   ?: 3*  [%record-type-check #<record-type qif-file> #f]
   ?: 4   (or (eq? rtd #) (scm-error # "%record-type-check" ...))
   ?: 5*  [eq? #<record-type qif-file> ...
   ?: 6*   [record-type-descriptor #f]
   ?: 7    (if (struct? obj) (struct-vtable obj) (error (quote not-a-record) obj))
     ...
   ?: 8    [scm-error misc-error #f "~A ~S" (not-a-record #f) #f]


This happens because the druid overwrites the wind->selected_file when an entry of the loaded files list is selected or deselected.  In the latter case selected_file is set to SCM_BOOL_F.  This does not mix well with gnc_ui_qif_import_default_acct_next_cb.

It would be cool if the "unload file" and "back" buttons would be grayed out when no file is selected anyway.