[Kivy] Initialize widget value from Python

[j.crater]

I can't for the life of me get my code to initialize a widget (in a kv file) from Python code on program startup.
Simplified Python code is here:

inital_text = "init text"

class MainApp(App):

    def initialize_widgets(self):
        self.root.ids.my_label.text = initial_text

if __name__ == '__main__':
    MainApp().run()
    MainApp.initialize_widgets(App)

And kv file:

Label:
    id: my_label
    text: "default text" 

When I run the program, the label says "default text". When quitting it, I get the error:

Error:
line 5, in initialize_widgets
self.root.ids.my_label.text = initial_text
AttributeError: type object 'App' has no attribute 'root'

I have tried various workarounds, many quite desperate ones, with no success. Help is much appreciated.