DropDowns crashing with weak reference error in kivy python

Question

I have a grid of CustomDropDown widgets which do not work in the same way everytime. But everytime I get this common error:

ReferenceError: weakly-referenced object no longer exists

My python code is as follows:

    from kivy.app import App
    from kivy.uix.label import Label
    from kivy.uix.boxlayout import BoxLayout
    from kivy.uix.button import Button
    from kivy.uix.dropdown import DropDown
    from kivy.lang import Builder
    from kivy.uix.relativelayout import RelativeLayout

    Mess = {
            'mess1': 'RR - SI',
            'mess2': 'VTH - SI',
            'mess3': 'SK - SI',
            'mess4': 'CV - SI',
            'mess5': 'CR - NI',
            'mess6': 'AD - NI',
            'mess7': 'MD - NI',
            'mess8': 'MDJ - NI',
            'mess9': 'FC - HM'  
    }
    Builder.load_string('''
    <ChoiceLayout>
        orientation:'vertical'
        Label:
            text: "Mess Registration for Jul-Nov 2016"
            size_hint: (None,None)
            font_size:'48dp'
            size_hint_y:1
            pos_hint: {'center_x': .5}
        BoxLayout:
            orientation:'horizontal'
            size_hint_y:2
            Label:
                text: 'NAME : {}'.format('MyName')
                size_hint: (None,None)
                font_size:'36dp'
                pos_hint: {'left': 1}
            Label:
                text: 'ROLL NUMBER : {}'.format('CS22A565')
                size_hint: (None,None)
                font_size:'36dp'
                pos_hint: {'right': 1}
        BoxLayout:
            orientation:'vertical'
            size_hint_y: 4
            BoxLayout:
                orientation:'horizontal'
                CustomDropDown:
                    id: dropdown1
                CustomDropDown:
                    id: dropdown2
                CustomDropDown:
                    id: dropdown3
            BoxLayout:
                orientation:'horizontal'
                CustomDropDown:
                    id: dropdown4
                CustomDropDown:
                    id: dropdown5
                CustomDropDown:
                    id: dropdown6
            BoxLayout:
                orientation:'horizontal'
                CustomDropDown:
                    id: dropdown7
                CustomDropDown:
                    id: dropdown8
                CustomDropDown:
                    id: dropdown9
        Button:
            text: 'Submit'
    <CustomDropDown>
        orientation:'vertical'
        Button:
            id: btn
            text:'click me'
            on_release: dropdown.open(self)
            size_hint_y: None
            height: '24dp'
        DropDown:
            id: dropdown
            on_parent: self.dismiss()
            on_select: btn.text= '{}'.format(args[1])
            size_hint_y: None
            height: '24dp'
        ''')

    class ChoiceLayout(BoxLayout):
        pass
    class CustomDropDown(BoxLayout):
        def build(self):
            tempMess = Mess
            for mess,mname in tempMess:
                mBtn = Button(text = mname,on_release = self.dropdown.select(mess))
                self.dropdown.add_widget(mBtn)
                del tempMess[mess]
        def selected(args):
            self.ids['btn'].text='{}'.format(args)


    class RegisterApp(App):
        def build(self):
            return ChoiceLayout()


    if __name__ == '__main__':
        RegisterApp().run()

So, the things that possibly I want to do is:

• Enable the CustomDropDowns one by one.
• The Weak reference error removed.

And the log in the console is:

 Traceback (most recent call last):
   File "prog.py", line 100, in <module>
     RegisterApp().run()
   File "/usr/lib/python2.7/dist-packages/kivy/app.py", line 828, in run
     runTouchApp()
   File "/usr/lib/python2.7/dist-packages/kivy/base.py", line 487, in runTouchApp
     EventLoop.window.mainloop()
   File "/usr/lib/python2.7/dist-packages/kivy/core/window/window_sdl2.py", line 619, in mainloop
     self._mainloop()
   File "/usr/lib/python2.7/dist-packages/kivy/core/window/window_sdl2.py", line 362, in _mainloop
     EventLoop.idle()
   File "/usr/lib/python2.7/dist-packages/kivy/base.py", line 330, in idle
     self.dispatch_input()
   File "/usr/lib/python2.7/dist-packages/kivy/base.py", line 315, in dispatch_input
     post_dispatch_input(*pop(0))
   File "/usr/lib/python2.7/dist-packages/kivy/base.py", line 281, in post_dispatch_input
     wid.dispatch('on_touch_up', me)
   File "kivy/_event.pyx", line 718, in kivy._event.EventDispatcher.dispatch (kivy/_event.c:7699)
   File "/usr/lib/python2.7/dist-packages/kivy/uix/behaviors/button.py", line 138, in on_touch_up
     self.dispatch('on_release')
   File "kivy/_event.pyx", line 714, in kivy._event.EventDispatcher.dispatch (kivy/_event.c:7654)
   File "kivy/_event.pyx", line 1224, in kivy._event.EventObservers.dispatch (kivy/_event.c:13497)
   File "kivy/_event.pyx", line 1108, in kivy._event.EventObservers._dispatch (kivy/_event.c:12329)
   File "/usr/lib/python2.7/dist-packages/kivy/lang.py", line 1557, in custom_callback
     exec(__kvlang__.co_value, idmap)
   File "<string>", line 57, in <module>
   File "kivy/weakproxy.pyx", line 19, in kivy.weakproxy.WeakProxy.__getattr__ (kivy/weakproxy.c:1143)
   File "kivy/weakproxy.pyx", line 15, in kivy.weakproxy.WeakProxy.__ref__ (kivy/weakproxy.c:1042)
 ReferenceError: weakly-referenced object no longer exists