Bad loading of a KV file

Question

When I run the program, the file is supposedly read correctly, but when it opens, only a button appears when a basic login interface should appear.

I tried using this code, I think the error is in the method build (function)

Now I ussing this code:

def build(self):
       # Crea un ScreenManager
        sm = ScreenManager()

        # Detecta la plataforma
        platform = self.detect_platform()

        # Carga el archivo KV de la pantalla de inicio de sesión según la plataforma
        kvfile = f"login{platform}.kv" if platform != "other" else "other_screen.kv"
        try:
            Builder.load_file(kv_file)
            print("Archivo KV cargado exitosamente")
        except Exception as e:
            print(f"Error al cargar archivo KV: {e}")
            return # Detiene la ejecución del método

        # Crea una instancia de LoginScreen
        login_screen = LoginScreen(name="login")
        sm.add_widget(login_screen)

        return sm

I was using the next code before and it worked fine. It worked fine, but when trying to load another KV file it couldn't, so I decided to change it to the current code:

def build(self):
        # Identifica el tipo de dispositivo
        device_type = "mobile" if platform == "android" or platform == "ios" else "pc"

        # Carga el archivo KV de la interfaz principal
        main_screen = Builder.loadfile(f"login{device_type}.kv")

        # Guarda el tipo de dispositivo
        self.device_type = device_type

        # Devuelve la interfaz principal
        return LoginScreen