from kivymd.app import MDApp
from kivymd.uix.screen import Screen
from kivymd.uix.button import MDFloatingActionButton

class SimpleApp(MDApp):
    def build(self):
        screen = Screen()
        btn_flat = MDFloatingActionButton(icon='account-circle',
            pos_hint={'center_x': 0.5, 'center_y': 0.5})
        screen.add_widget(btn_flat)
        return screen

SimpleApp().run()