Problems with grid-layout

I'm trying to implement a capi interface which has a very dynamic
grid-layout as one of its parts. I set the layout-description of this
grid-layout this way (where slot-input-pane generates two sub-panes,
one is a column-layout consisting of a title-pane and a button and one
is either a grid-layout with complex contents or an empty title-pane):

(defmethod make-edit-lists ((csf customer-select-form))
  (with-slots (edit edit-lists edit-main edit-buttons edited-customer) csf
    (let ((alist-panes (loop for things in (customer-alists edited-customer)
                             for slot = (first things)
                             append (slot-input-pane csf slot nil))))
      (setf (layout-description edit-lists) alist-panes))))

This almost works, but sometimes the program crashes while doing
the (setf layout-description), and I end up with a backtrace like
the one below.

Any ideas?

Error: In a call to ARRAY-DIMENSION: NIL is not of type ARRAY.
  1 (abort) Return to event loop.

Type :b for backtrace, :c <option number> to proceed,  or :? for other options

CL-USER 1 : 1 > :b
Call to (METHOD CAPI:CALCULATE-CONSTRAINTS (CAPI:GRID-LAYOUT))
Call to (METHOD CAPI::CALCULATE-INTERNAL-CONSTRAINTS (CAPI::SIMPLE-ELEMENT))
Call to CLOS::NEXT-METHOD-CALL-1
Call to (METHOD CAPI::CALCULATE-INTERNAL-CONSTRAINTS :AROUND (CAPI:SIMPLE-PANE))
Call to (METHOD CAPI::CALCULATE-EXTERNAL-CONSTRAINTS (CAPI::SIMPLE-ELEMENT))
Call to CLOS::NEXT-METHOD-CALL-1
Call to CLOS::NEXT-METHOD-CALL-1
Call to (METHOD CAPI::CALCULATE-EXTERNAL-CONSTRAINTS :AROUND (CAPI:SIMPLE-PANE))
Call to CAPI-INTERNALS:GET-CONSTRAINTS
Call to (SUBFUNCTION 3 (METHOD CAPI:CALCULATE-CONSTRAINTS (CAPI:GRID-LAYOUT)))
Call to CAPI::CALCULATE-GRID-CONSTRAINTS-1
Call to (METHOD CAPI:CALCULATE-CONSTRAINTS (CAPI:GRID-LAYOUT))
Call to (METHOD CAPI::CALCULATE-INTERNAL-CONSTRAINTS (CAPI::SIMPLE-ELEMENT))
Call to CLOS::NEXT-METHOD-CALL-1
Call to (METHOD CAPI::CALCULATE-INTERNAL-CONSTRAINTS :AROUND (CAPI:SIMPLE-PANE))
Call to (METHOD CAPI::CALCULATE-EXTERNAL-CONSTRAINTS (CAPI::SIMPLE-ELEMENT))
Call to CLOS::NEXT-METHOD-CALL-1
Call to (METHOD CAPI::CALCULATE-EXTERNAL-CONSTRAINTS :AROUND (CAPI::SIMPLE-ELEME
NT))
Call to CLOS::NEXT-METHOD-CALL-1
Call to (METHOD CAPI::CALCULATE-EXTERNAL-CONSTRAINTS :AROUND (CAPI:SIMPLE-PANE))
Call to CAPI-INTERNALS:GET-CONSTRAINTS
Call to CAPI::PANE-UPDATE-GEOMETRY-OF-CHILDREN
Call to (METHOD CAPI:MAP-PANE-CHILDREN (CAPI:LAYOUT T))
Call to (METHOD CAPI::CHANGE-CHILDREN-GEOMETRY (CAPI::PANE-WITH-CHILDREN))
Call to (METHOD CAPI:MAP-PANE-CHILDREN (CAPI:LAYOUT T))
Call to (METHOD CAPI::CHANGE-CHILDREN-GEOMETRY (CAPI::PANE-WITH-CHILDREN))
Call to (METHOD CAPI:MAP-PANE-CHILDREN (CAPI:LAYOUT T))
Call to (METHOD CAPI::CHANGE-CHILDREN-GEOMETRY (CAPI::PANE-WITH-CHILDREN))
Call to (METHOD CAPI:MAP-PANE-CHILDREN (CAPI:LAYOUT T))
Call to (METHOD CAPI::CHANGE-CHILDREN-GEOMETRY (CAPI::PANE-WITH-CHILDREN))
Call to (METHOD CAPI::CHANGE-CHILDREN-GEOMETRY (CAPI::PANE-WITH-CHILDREN))
Call to CAPI::DO-REDISPLAY
Call to (METHOD CAPI::REDISPLAY (CAPI::PANE-WITH-CHILDREN))
Call to CLOS::NEXT-METHOD-CALL-1
Call to (METHOD CAPI-INTERNALS:CAPI-GEOMETRY-CHANGE-CALLBACK (CAPI:INTERFACE T T
 T T T))
Call to MP::PROCESS-ALL-EVENTS
Call to CAPI-MOTIF-LIBRARY::DO-DESTROY
Call to CAPI::EXECUTE-WITH-INTERFACE-INTERNAL
Call to CAPI-MOTIF-LIBRARY::PERFORM-WINDOW-DESTROY
Call to (METHOD CAPI:MAP-PANE-CHILDREN (CAPI:LAYOUT T))
Call to (METHOD CAPI::IMMEDIATE-DESTROY :AFTER (CAPI::PANE-WITH-CHILDREN))
Call to (METHOD CAPI:DESTROY (CAPI:SIMPLE-PANE))
Call to (METHOD CAPI:MAP-PANE-CHILDREN (CAPI:LAYOUT T))
Call to CAPI::LAYOUT-INSTALL-PANES
Call to (METHOD CAPI::MAYBE-INSTALL-DESCRIPTION (CAPI:LAYOUT))
Call to (METHOD CAPI::REDO-LAYOUT (CAPI:LAYOUT))
Call to (METHOD CAPI::INVALIDATE-PANE-CONSTRAINTS-WITH-CONTROL-OVER-REDISPLAY (C
API:LAYOUT))
Call to CAPI::INVALIDATE-PANE-CONSTRAINTS-WITH-REDISPLAY
Call to CAPI::REDISPLAY-LAYOUT-CREATING-UNMAPPED-CHILDREN
Call to (METHOD CAPI::SET-LAYOUT-DESCRIPTION (CAPI:LAYOUT T))
Call to CAPI::EXECUTE-CALLBACK
Call to CAPI-INTERNALS:LOOP-PROCESS-EVENTS
Call to CAPI::INTERFACE-EVENT-LOOP
Call to CAPI::INITIALIZE-AND-PROCESS-EVENTS
Call to (METHOD CAPI-LIBRARY:INTERFACE-PROCESS-WRAPPER (CAPI-MOTIF-LIBRARY::SCRE

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  (espen)