title: Custom event : TypeError: 'NoneType' object is not subscriptable
body: <p>I use Kivy 1.8.0 and I try to use the custom event to manage a yesNo popup. This code was found on the internet. I have find only this sample and I want to modify this code to make multiple popup management but I have error when run it.</p>
  
  <pre><code>import kivy
  from kivy.app import App
  from kivy.lang import Builder
  from kivy.uix.gridlayout import GridLayout
  from kivy.uix.popup import Popup
  from kivy.properties import StringProperty
  
  Builder.load_string('''
  &lt;ConfirmPopup&gt;:
      cols: 1
          Label:
              text: root.text
          GridLayout:
              cols: 2
              size_hint_y: None
              height: '44sp'
              Button:
                  text: 'Yes'
                  on_release: root.dispatch('on_reponse_user','yes')
              Button:
                  text: 'No'
                  on_release: root.dispatch('on_reponse_user', 'no')
  ''')
  </code></pre>
  
  <p>I don't know if the problem is in the string for kv or is in the code.</p>
  
  <pre><code>class ConfirmPopup(GridLayout):
      text = StringProperty('')
  
      def __init__(self,**kwargs):
          self.register_event_type('on_reponse_user')
          super(ConfirmPopup,self).__init__(**kwargs)
  
      def on_reponse_user(self, instance, answer):
          pass    
  
  
  class PopupTest(App):
      def build(self):
          content = ConfirmPopup(text='Do You Love Kivy?')
          content.bind(on_reponse_user=self.on_answer_callback)
          self.popup = Popup(title="Answer Question",
                             content=content,
                             size_hint=(None, None),
                             size=(480,400),
                             auto_dismiss= False)
          self.popup.open()
  
      def on_answer_callback(self, instance, answer):
          print("USER ANSWER: " , repr(answer))
          self.popup.dismiss()
  
  if __name__ == '__main__':
      PopupTest().run()
  </code></pre>
  
  <p>When I run this code I have This error :</p>
  
  <pre><code> Traceback (most recent call last):
     File "F:\Kivy-1.8.0\test_event.py", line 24, in &lt;module&gt;
       ''')
     File "F:\Kivy-1.8.0\kivy\kivy\lang.py", line 1491, in load_string
       parser = Parser(content=string, filename=fn)
     File "F:\Kivy-1.8.0\kivy\kivy\lang.py", line 1049, in __init__
       self.parse(content)
     File "F:\Kivy-1.8.0\kivy\kivy\lang.py", line 1122, in parse
       objects, remaining_lines = self.parse_level(0, lines)
     File "F:\Kivy-1.8.0\kivy\kivy\lang.py", line 1271, in parse_level
       if current_property[:3] == 'on_':
   TypeError: 'NoneType' object is not subscriptable
  </code></pre>
  
  <p>I don't know why I have an error with the custom event. Like the documentation in Kivy it is possible to add custom event.</p>