How do I fix this? ValueError: KivyMD: App object must be inherited from `kivymd.app.MDApp`

Question

I just cannot understand why it doesn't, I'm trying to create this simple program but it gives me this error. my code is:

main.py

from kivy.app import App
from kivymd.theming import ThemeManager

class Mainapp(App):
        theme_cls = ThemeManager()

Mainapp().run()

helper.kv

NavigationLayout:

   MDNavigationDrawer:

   ScreenManager:

Answer 1

You need to change:

class Mainapp(App):

to:

class Mainapp(MDApp):