Even so, when fixing it as you kindly indicated, it still bombs --
The Goatee editable area object, which contains the buffer, is not initialized until the first redisplay happens, so it is impossible to access the buffer before then. Hence, you cannot use (setf gadget-value) on a text-field or text-editor gadget in McCLIM before your command loop is running and the gadget has been redisplayed at least once. I'm pretty sure this is a bug in McCLIM (if anyone wants to fix it, look at the definition of `handle-repaint' for `text-field-pane' in gadget.lisp line 2542).