How to implement in Python Flet a BottomAppBar

I have an extra Page and want to add an BottomAppBar. Here the Code:

Main and Start are Different files, but start can use pageIn the Documentation it shows, that i just have to add the page.BottomAppBar. I tried to add it in the main(), this was working

#Main:import fletfrom flet import *from pages.start import StartPagedef main(page: Page):    app = Startgage(show_game_page ,page)    page.add(app)if __name__ == "__main__":    flet.app(        target=main,        assets_dir='assets')#Start:class StartPage(UserControl):    def __init__(self, show_game_page, page):        super().__init__()        self.show_game_page = show_game_page        self.page = page    def build(self):        self.page.bottom_appbar = BottomAppBar(            content=self.player_row,            bgcolor=colors.GREY_400,            visible=True,        )        #[...]

I just want to see the BottomAppBar