popup attaches to window behind all other widgets

gistfile1.txt

    def build(self):

        l= Builder.load_file('grd7.kv')
        m=  Builder.load_file('studentlist3.kv')
        carousel = Carousel(direction='right')
        carousel.add_widget(l)
        carousel.add_widget(m)
        popup = Popup(title='Please select a waypoint',
        content = Button(text='close window'),
        size_hint=(None, None), size=(400, 400))
        #carousel.add_widget(popup, 1000)
        popup.open()
        #print 'ids are', m.ids
        self.m = m
        self.l = l
        return carousel