Вывести на экран (label) текста

пытаюсь вывести на экран текст но все бесполезно, помогите с кодом.

from kivy.app import App
from kivy.lang import Builder
from kivy.properties import ObjectProperty, StringProperty
from kivymd.theming import ThemeManager
# from kivymd.navigationdrawer import NavigationDrawer
from navigationdrawer import NavigationDrawer
from kivy.uix.image import Image
from kivy.uix.label import Label
from kivy.clock import Clock


main_widget_kv = '''
#:import Toolbar kivymd.toolbar.Toolbar
#: import NoTransition kivy.uix.screenmanager.NoTransition

BoxLayout:
    orientation: 'vertical'
    Toolbar:
        id: toolbar
        title: 'Welcome'
        background_color: app.theme_cls.primary_dark
        left_action_items: [['menu', lambda x: app.nav_drawer.toggle()]]
        #right_action_items: [['more-vert', lambda x: app.raised_button.open(self.parent)]]
    Label:





'''

class NavigateApp(App):
    theme_cls = ThemeManager()
    nav_drawer = ObjectProperty()


    def build(self):
        main_widget = Builder.load_string(main_widget_kv)
        #self.nav_drawer = Navigator()
        return main_widget

NavigateApp().run()