[TraitsGUI] ChainedWizard _next_wizard_changed() method

I would like to create a chained wizard with a first page where I
select from a list of other wizards and this determines the next
wizard.  Please find attached an example of what I am trying to
achieve.

The problem that I have is that when the next_wizard trait of a
ChainedWizard is changed the _next_wizard_changed() method calls
_create_buttons().

    def _next_wizard_changed(self, old, new):
        """ Handle the next wizard being changed. """

        if new is not None:
            self.controller.next_controller = new.controller

#        if self.control is not None:
#            self._create_buttons(self.control)
#            self._update()

        return

As I understand it, _create_buttons() is normally called from
_create_contents() and returns a sizer which is then added to the main
sizer.  The _next_wizard_changed() method does not use the returned
sizer and the created buttons are placed in the top-left corner of the
dialog.  The main problem is that there is no end to the subsequent
pages of the wizard and I get the following traceback if I keep
clicking Next.

Traceback (most recent call last):