code: def get_builder_toplevel(self, builder):
          
          toplevel = builder.get_object(self.toplevel_name)
          if not GObject.type_is_a(toplevel, Gtk.Window):
              toplevel = None
          if toplevel is None:
              toplevel = get_first_builder_window(builder)
          return toplevel
docs: Get the toplevel widget from a Gtk.Builder file.
  
          The main view implementation first searches for the widget named as
          self.toplevel_name (which defaults to "main". If this is missing, or not
          a Gtk.Window, the first toplevel window found in the Gtk.Builder is
          used.