human
Solve the following python programming problem given the following code:
def add_close_button(self, cb = None, * userdata) : 
    assert (self._has_close_button == False) 
    self._has_close_button = True 
    bn = gtk.Button () 
    bn.set_property ('relief', gtk.RELIEF_NONE) 
    bn.set_property ('can-focus', False) 
    bn.set_image (gtk.image_new_from_stock (gtk.STOCK_CLOSE, gtk.ICON_SIZE_SMALL_TOOLBAR)) 
    bn.modify_bg (gtk.STATE_PRELIGHT, gtk.gdk.color_parse ('#E1D18C')) 
    bn.modify_bg (gtk.STATE_NORMAL, gtk.gdk.color_parse ('#E1D18C')) 
    self._hbox1.pack_end (self._mka (bn), False, False, 2) 
    def on_click(* args) : 
        if cb : 
            cb (* userdata) 
self._after_button_pressed.fire () 
bn.connect ('clicked', cb) 

    
    

    Fix the buggy line: bn.connect ('clicked', cb)
assistant
#Here is Python program to fix the buggy line: bn.

bn.connect ('clicked', def on_click()