title: Kivy ScrollView exception raise Exception('ScrollView accept only one widget')
body: <p>Using the following code, after I input the information into the text boxes in my app, the offset_back returns a list and self.main, the id used in my .kv file to create a scrollview 
  adds the layout but when I try to re-enter data a second time this happens:     "raise Exception('ScrollView accept only one widget')"</p>
  
  <p>Do I need to reset self.main some way and if so how can I achieve this?
  Thanks.</p>
  
  <pre><code>def set_back_offset(self):
      layout = GridLayout(cols=1, spacing=10, size_hint_y=None)
      layout.bind(minimum_height=layout.setter('height'))
  
       # catch empty box input which is a str, as Decimal will throw error.
      if self.initial_bet.text == str() or self.initial_odds.text == str():
          pop_warning().open()
       #make sure entered unicode text is numeric, if not throw pop-up warning
      elif not unicode.isnumeric(self.initial_bet.text) or not unicode.isnumeric(self.initial_odds.text):
              pop_warning().open()
  
      else:# all is good, create widgets
          for details in offset_back(Decimal(self.initial_bet.text), Decimal(self.initial_odds.text)):
              btn = Button(text=str(details[0]), size_hint_y=None, height=40,background_color= (255,0,0,1))
              layout.add_widget(btn)
          self.main.add_widget(layout)
  </code></pre>
  
  <p>`</p>