title: How to reference to an instance created in KV file
body: <p>I want to call a method <code>add_category_to_tree()</code> from <code>TreeCategory</code> class, when pressing SAVE button in <code>AddCategoryPopup</code> class. However, I have problems how to reference to the TreeCategory instance, which is created in KV file.
  I was trying to search solution, but nothing works. Currently i get  <code>AttributeError: 'super' object has no attribute '__getattr__'</code> error.</p>
  
  <p>How should I do this properly ? Thank you for help</p>
  
  <pre><code>class TreeCategory(TreeView):
      def __init__(self, **kwargs):
          super(TreeCategory, self).__init__(**kwargs)
  
      def add_category_to_tree(self, name):
          self.add_node(TreeViewLabel(text = name.upper()))
  
  
  class AddCategoryPopup(Popup):
  
      def save(self):
          self.ids.tree.add_category_to_tree(self.ids.entry.text) # ????
          db.adding_to_db('kategorie', 'nazwa', self.ids.entry.text)
          self.dismiss()
  
      def close(self):
          self.dismiss()
  
  class MainScreen(BoxLayout):
      tree = ObjectProperty(None)
  
      def add_category_button(self):
          popup = AddCategoryPopup(title = 'Dodawanie nowej kategorii')
          return popup.open()
  
  
  class GuiCookBookApp(App):
      def build(self):
          self.title = "Książka kucharska"
          return MainScreen()
  
  
  if __name__ == "__main__":
      db = DatabaseManager("cookbook.sqlite")
      GuiCookBookApp().run()
  </code></pre>
  
  <p>KV file:</p>
  
  <pre><code>&lt;AddCategoryPopup&gt;:
      BoxLayout:
          orientation: 'vertical'
          TextInput:
              id: entry
              multiline: False
              hint_text: 'Podaj nazwę kategorii...'
          BoxLayout:
              orientation: 'horizontal'
              Button:
                  text: 'SAVE'
                  on_press: root.save()
              Button:
                  text: 'CANCEL'
                  on_press: root.close()
  
  &lt;MainScreen&gt;:
      orientation: "vertical"
      display: entry
      tree: tree
  
      BoxLayout:
          id: menu
          size_hint_y: .1
  
          Button:
              text: 'Dodaj kategorię'
              on_press: root.add_category_button()
  
      BoxLayout:
          id: recipe_view
          orientation: "horizontal"
  
          TreeCategory:
              id: tree
              hide_root: True
              size_hint: .25, 1
  </code></pre>