code: def _create_notebook_page(self, assistant):
          
          #frame = self._create_frame()
          grid_lang = self.gui_helper.create_gtk_grid()
          scrolled_window = self.gui_helper.create_scrolled_window(grid_lang)
          row = 0
          column = 0
          scrolled_window.main_assistant, sub_as = assistant.get_subassistant_tree()
          for ass in sorted(sub_as, key=lambda x: x[0].fullname.lower()):
              if column > 2:
                  row += 1
                  column = 0
              if not ass[1]:
                  # If assistant has not any subassistant then create only button
                  self.gui_helper.add_button(grid_lang, ass, row, column)
              else:
                  # If assistant has more subassistants then create button with menu
                  self.gui_helper.add_submenu(grid_lang, ass, row, column)
              column += 1
  
          # Install More Assistants button
          if column > 2:
              row += 1
              column = 0
          self.gui_helper.add_install_button(grid_lang, row, column)
          column += 1
  
          if row == 0 and len(sub_as) < 3:
              while column < 3:
                  btn = self.gui_helper.create_button(style=Gtk.ReliefStyle.NONE)
                  btn.set_sensitive(False)
                  btn.hide()
                  grid_lang.attach(btn, column, row, 1, 1)
                  column += 1
          return scrolled_window
docs: This function is used for create tab page for notebook.
          Input arguments are:
          assistant - used for collecting all info about assistants and subassistants