[options.scm] gnc:make-budget-option stores guid instead of object

because object may become stale if UI is used to delete it, leading to
stale pointer and segfault. storing guid is safer, and will return
null if budget is deleted.

libgnucash/app-utils/options.scm

@@ -470,6 +470,13 @@ the option '~a'."))
 	 (selection-budget initial-budget)
          )
 
+
+    (define (convert-to-guid item)
+      (if (string? item) item (gncBudgetGetGUID item)))
+
+    (define (convert-to-budget item)
+      (if (string? item) (gnc-budget-lookup item (gnc-get-current-book)) item))
+
     (gnc:make-option
      section 
      name 
@@ -478,12 +485,12 @@ the option '~a'."))
      documentation-string
 
      ;; getter -- Return a budget pointer
-     (lambda () 
-       selection-budget)
+     (lambda ()
+       (convert-to-budget selection-budget))
 
      ;; setter -- takes a budget
      (lambda (x)
-       (set! selection-budget x))
+       (set! selection-budget (convert-to-guid x)))
 
      ;; default-getter
      ;; Default now is #f so saving is independent of book-level default