code: def init_layout(self):
          
          super(AndroidRadioGroup, self).init_layout()
          d = self.declaration
          w = self.widget
          if d.checked:
              self.set_checked(d.checked)
          else:
              #: Check if any of the children have "checked = True"
              for c in d.children:
                  if c.checked:
                      d.checked = c
  
          w.setOnCheckedChangeListener(w.getId())
          w.onCheckedChanged.connect(self.on_checked_changed)
docs: Set the checked state after all children have
          been populated.