Error Code : “KivyMD: App object must be initialized before loading ”

Question

I am receiving the Error: "KivyMD: App object must be initialized before loading " when I am trying to add a kivymd button (MDRaisedButton).

I tried debugging, the issue is when I add the BoxLayout under the DateScreen, I just don't find any information on how to fix it.

How should one initialized app object before loading?

import kivy
kivy.require("1.11.1")
from kivy.app import App
from kivy.lang import Builder
from kivy.clock import Clock
from kivy.uix.screenmanager import Screen
from kivymd.theming import ThemeManager


class WelcomeScreen(Screen):  # Defines WelcomeScreen instance as a screen widget.
    pass

class DateScreen(Screen):  # Defines DateScreen instance as a screen widget.
    pass

class ResultScreen(Screen):  # Defines ResultScreen instance as a screen widget.
    pass

root_widget = Builder.load_string("""
ScreenManager:
    WelcomeScreen:
    DateScreen:
    ResultScreen:
<WelcomeScreen>:
    _welcome_screen_text_: welcome_screen
    name: 'my_welcome_screen'
    Label:
        id: welcome_screen
        Image: 
            source: 'Cheers.jpg'
            size: 200, 200
            center: self.parent.center

<DateScreen>:
    _date_screen_text_: date_screen
    name: 'my_date_screen'
    Label:
        id: date_screen
        text: "This is the date selection screen"
    BoxLayout:
        size_hint: 0.1, 0.5
        pos_hint: {'center_x': 0.5, 'center_y': 0.5}
        MDRaisedButton:
            text: 'MDButton1'
<ResultScreen>:
    _result_screen_text_: result_screen
    name: 'my_result_screen'
    Label:
        id: result_screen
        text: "This is where the result will be displayed"
""")

class MainApp(App):

    theme_cls = ThemeManager()

    def build(self):
        self.theme_cls.theme_style = "Dark"
        Clock.schedule_once(self.screen_switch_one, 36)  # clock callback for the first screen
        Clock.schedule_once(self.screen_switch_two, 4)  # clock callback for the second screen
        return root_widget


    def screen_switch_one(a, b):
        root_widget.current = 'my_welcome_screen'  

    def screen_switch_two(a, b):
        root_widget.current = 'my_date_screen' 

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

Please help!

Answer 1

I believe you need to call Builder after the App is initialized (as mentioned in your error). To do that you can save your kv in a string, then call Builder inside the build() method. Like this:

import kivy
from kivymd.app import MDApp

kivy.require("1.11.1")
from kivy.lang import Builder
from kivy.clock import Clock
from kivy.uix.screenmanager import Screen


class WelcomeScreen(Screen):  # Defines WelcomeScreen instance as a screen widget.
    pass

class DateScreen(Screen):  # Defines DateScreen instance as a screen widget.
    pass

class ResultScreen(Screen):  # Defines ResultScreen instance as a screen widget.
    pass

kv = """
ScreenManager:
    WelcomeScreen:
    DateScreen:
    ResultScreen:
<WelcomeScreen>:
    _welcome_screen_text_: welcome_screen
    name: 'my_welcome_screen'
    Label:
        id: welcome_screen
        Image: 
            source: 'Cheers.jpg'
            size: 200, 200
            center: self.parent.center

<DateScreen>:
    _date_screen_text_: date_screen
    name: 'my_date_screen'
    Label:
        id: date_screen
        text: "This is the date selection screen"
    BoxLayout:
        size_hint: 0.1, 0.5
        pos_hint: {'center_x': 0.5, 'center_y': 0.5}
        MDRaisedButton:
            text: 'MDButton1'
<ResultScreen>:
    _result_screen_text_: result_screen
    name: 'my_result_screen'
    Label:
        id: result_screen
        text: "This is where the result will be displayed"
"""


class MainApp(MDApp):

    def build(self):
        self.theme_cls.theme_style = "Dark"
        self.root_widget = Builder.load_string(kv)
        Clock.schedule_once(self.screen_switch_one, 36)  # clock callback for the first screen
        Clock.schedule_once(self.screen_switch_two, 4)  # clock callback for the second screen
        return self.root_widget


    def screen_switch_one(self, dt):
        self.root_widget.current = 'my_welcome_screen'

    def screen_switch_two(self, dt):
        self.root_widget.current = 'my_date_screen'

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