Update the color scheme of the preview editor and adds text.

        Note
        ----
        'index' is needed, because this is triggered by a signal that sends
        the selected index.

def update_preview(self, index=None, scheme_name=None):
        """
        Update the color scheme of the preview editor and adds text.

        Note
        ----
        'index' is needed, because this is triggered by a signal that sends
        the selected index.
        """
        text = ('"""A string"""\n\n'
                '# A comment\n\n'
                '# %% A cell\n\n'
                'class Foo(object):\n'
                '    def __init__(self):\n'
                '        bar = 42\n'
                '        print(bar)\n'
                )
        show_blanks = CONF.get('editor', 'blank_spaces')
        update_scrollbar = CONF.get('editor', 'scroll_past_end')
        if scheme_name is None:
            scheme_name = self.current_scheme
        self.preview_editor.setup_editor(linenumbers=True,
                                         markers=True,
                                         tab_mode=False,
                                         font=get_font(),
                                         show_blanks=show_blanks,
                                         color_scheme=scheme_name,
                                         scroll_past_end=update_scrollbar)
        self.preview_editor.set_text(text)
        self.preview_editor.set_language('Python')