move a test

xenomorph/xenomorph/struct.rkt

@@ -50,6 +50,9 @@ https://github.com/mbutterick/restructure/blob/master/src/Struct.coffee
     (super-new)
     (init-field [(@fields fields)])
     
+    (when @fields (unless (dict? @fields)
+                    (raise-argument-error '+xstruct "dict" @fields)))
+
     (define/augride (xxdecode port parent [len 0])
       ;; xstruct-setup and xstruct-parse-fields are separate to cooperate with VersionedStruct
       (define decoded-hash
@@ -100,8 +103,6 @@ https://github.com/mbutterick/restructure/blob/master/src/Struct.coffee
                            (unless (symbol? (car kv))
                              (raise-argument-error '+xstruct "symbol" (car kv)))
                            (apply cons kv)))
-  (unless (dict? fields)
-    (raise-argument-error '+xstruct "dict" fields))
   (new class [fields fields]))
 
 (module+ test