Высвечивается просто белый экран

import json
from kivymd.app import MDApp
from kivy.lang.builder import Builder
from kivy.uix.screenmanager import ScreenManager, Screen
from kivymd.uix.button import MDIconButton, MDRaisedButton
from kivymd.uix.card import MDCard
from kivymd.uix.textfield import MDTextField
from kivymd.uix.gridlayout import MDGridLayout
from kivymd.uix.label import MDLabel
from kivymd.uix.list import MDList, TwoLineListItem
from kivy.metrics import dp
from kivy.properties import ObjectProperty
from kivy.uix.boxlayout import BoxLayout
from functools import partial
import traceback
 
screen_helper = """ 
ScreenManager:
    MenuScreen:
    CreateScreen:
    StatsScreen:
    FlashcardsScreen:
 
<MenuScreen>:
    name: 'menu'
    MDGridLayout:
        id: group_list_layout
        cols: 1
        size_hint_y: None
        height: self.minimum_height
        pos_hint: {"top": 0.9}
    MDTopAppBar:
        title: "Answa"
        pos_hint: {"top": 1}
        elevation: 0
        md_bg_color: (34 / 255, 139 / 255, 34 / 255) 
        right_action_items: [["chart-bar-stacked", lambda x: root.on_right_action_items(x)]]
    MDFloatingActionButton:
        icon: "note-plus"
        md_bg_color: (34 / 255, 139 / 255, 34 / 255)
        pos: root.width - self.width - dp(16), dp(16)
        elevation: 0
        on_press: root.manager.current = 'create'
 
<CreateScreen>:
    name: 'create'
    MDTopAppBar:
        title: "Answa"
        pos_hint: {"top": 1}
        elevation: 0
        left_action_items: [["chevron-left", lambda x: root.on_left_action_items(x)]]
        right_action_items: [["content-save-check-outline", lambda x: root.save_to_json()]]
        md_bg_color: (34 / 255, 139 / 255, 34 / 255)
    MDTextField:
        id: group_name
        hint_text: "Enter name group..."
        mode: "fill"
        pos_hint: {"top": 0.9}
    ScrollView:
        size_hint: (1, 1)
        pos_hint: {"top": 0.8}
        GridLayout:
            id: card_container
            cols: 1
            size_hint_y: None
            height: self.minimum_height
    MDIconButton:
        icon: "plus-circle-outline"
        theme_icon_color: "Custom"
        icon_color: (34 / 255, 139 / 255, 34 / 255)
        icon_size: "54sp"
        pos: root.width - self.width - dp(16), dp(16)
        on_release: root.add_card()
 
<StatsScreen>:
    name: 'stats'
    MDTopAppBar:
        title: "Answa"
        pos_hint: {"top": 1}
        elevation: 0
        left_action_items: [["chevron-left", lambda x: root.on_left_action_items(x)]]  
        md_bg_color: (34 / 255, 139 / 255, 34 / 255)
    MDLabel:
        text: 'Upload'
        halign: 'center'
 
<FlashcardsScreen>:
    name: 'flashcards'
    MDTopAppBar:
        title: "Answa"
        pos_hint: {"top": 1}
        elevation: 0
        left_action_items: [["chevron-left", lambda x: root.on_left_action_items(x)]]  
        md_bg_color: (34 / 255, 139 / 255, 34 / 255)
    MDLabel:
        text: 'Upload'
        halign: 'center'
"""
 
class MenuScreen(Screen):
    group_list_layout = ObjectProperty()
 
    def __init__(self, **kwargs):
        super(MenuScreen, self).__init__(**kwargs)
        self.group_list_layout = MDGridLayout(cols=1, size_hint_y=None, height=dp(0))
 
    def on_pre_enter(self, *args):
        self.update_group_list()
 
    def on_right_action_items(self, instance):
        self.manager.current = 'stats'
 
    def update_group_list(self):
        self.group_list_layout.clear_widgets()
 
        try:
            with open("user_data.json", "r") as file:
                data = json.load(file)
 
            for group_data in data:
                group_name = group_data.get("group_name", "")
                if group_name:
                    card = MDCard(
                        orientation='vertical',
                        size_hint_y=None,
                        height=dp(80),
                        elevation=1,
                        on_release=partial(self.show_flashcards_screen, group_data)
                    )
 
                    box_layout = BoxLayout(padding=(dp(16), 0))
                    label = MDLabel(
                        text=group_name,
                        theme_text_color="Primary",
                        halign="left",
                        valign="center",
                    )
 
                    box_layout.add_widget(label)
                    card.add_widget(box_layout)
 
                    self.group_list_layout.add_widget(card)
 
        except FileNotFoundError as e:
            print(f"Error: {e}")
        except Exception as e:
            print(f"An error occurred: {traceback.format_exc()}")
 
    def show_flashcards_screen(self, group_data, *args):
        self.manager.current = 'flashcards'
        flashcards_screen = self.manager.get_screen('flashcards')
        flashcards_screen.group_data = group_data
        flashcards_screen.load_flashcards()
 
class CreateScreen(Screen):
    card_list_layout = ObjectProperty()
 
    def on_left_action_items(self, instance):
        self.manager.current = 'menu'
 
    def __init__(self, **kwargs):
        super(CreateScreen, self).__init__(**kwargs)
        self.card_data = []
 
    def add_card(self):
        card_container = self.ids.card_container
 
        new_card = MDCard(
            orientation='vertical',
            size_hint_y=None,
            height="80dp",
            elevation=1,
        )
 
        new_box_layout = MDGridLayout(
            cols=2,
            row_default_height="40dp",
            row_force_default=True,
            spacing='10dp',
            padding='10dp',
        )
 
        new_question_field = MDTextField(hint_text='Question')
        new_answer_field = MDTextField(hint_text='Answer')
 
        new_box_layout.add_widget(new_question_field)
        new_box_layout.add_widget(new_answer_field)
 
        close_button = MDIconButton(
            icon="close",
            theme_text_color="Custom",
            text_color=(1, 0, 0, 1),
            pos_hint={"center_x": 0.95, "center_y": 0.9},
        )
 
        close_button.bind(on_release=lambda x: self.remove_card(new_card, new_question_field, new_answer_field))
 
        new_card.add_widget(new_box_layout)
        new_card.add_widget(close_button)
 
        self.card_data.insert(0, {"question": new_question_field.text, "answer": new_answer_field.text})
 
        card_container.add_widget(new_card, index=0)
 
        new_question_field.text = ''
        new_answer_field.text = ''
 
    def update_card_container(self):
        card_container = self.ids.card_container
        card_container.clear_widgets()
 
        for card_data in self.card_data:
            new_card = MDCard(
                orientation='vertical',
                size_hint_y=None,
                height="80dp",
                elevation=1,
            )
 
            new_box_layout = MDGridLayout(
                cols=2,
                row_default_height="40dp",
                row_force_default=True,
                spacing='10dp',
                padding='10dp',
            )
 
            new_question_field = MDTextField(hint_text='Question', text=card_data.get('question', ''))
            new_answer_field = MDTextField(hint_text='Answer', text=card_data.get('answer', ''))
 
            new_box_layout.add_widget(new_question_field)
            new_box_layout.add_widget(new_answer_field)
 
            close_button = MDIconButton(
                icon="close",
                theme_text_color="Custom",
                text_color=(1, 0, 0, 1),
                pos_hint={"center_x": 0.95, "center_y": 0.9},
            )
 
            close_button.bind(on_release=lambda x: self.remove_card(new_card, new_question_field, new_answer_field))
 
            new_card.add_widget(new_box_layout)
            new_card.add_widget(close_button)
 
            card_container.add_widget(new_card, index=0)
 
    def remove_card(self, card, question_field, answer_field):
        card_container = self.ids.card_container
        card_container.remove_widget(card)
 
        for item in self.card_data:
            if item["question"] == question_field.text and item["answer"] == answer_field.text:
                self.card_data.remove(item)
                break
 
class FlashcardsScreen(Screen):
    def __init__(self, group_data=None, **kwargs):
        super().__init__(**kwargs)
        self.group_data = group_data
        self.layout = BoxLayout(orientation='vertical')
        self.flashcard_list = MDList()
        self.edit_button = MDRaisedButton(text='Edit Flashcards', on_release=self.edit_flashcards)
        self.study_button = MDRaisedButton(text='Study', on_release=self.study_flashcards)
        self.layout.add_widget(self.flashcard_list)
        self.layout.add_widget(self.edit_button)
        self.layout.add_widget(self.study_button)
        self.add_widget(self.layout)
 
    def on_pre_enter(self, *args):
        if self.group_data:
            self.load_flashcards()
 
    def load_flashcards(self):
        self.flashcard_list.clear_widgets()
        flashcards = self.group_data.get('flashcards', [])
        for flashcard in flashcards:
            text = f"Question: {flashcard['question']}\nAnswer: {flashcard['answer']}"
            item = TwoLineListItem(text=text)
            self.flashcard_list.add_widget(item)
 
    def edit_flashcards(self, instance):
        self.manager.current = 'create'
        create_screen = self.manager.get_screen('create')
        create_screen.group_name.text = self.group_data.get('group_name', '')
        create_screen.card_data = self.group_data.get('flashcards', [])
        create_screen.update_card_container()
 
    def study_flashcards(self, instance):
        if not self.group_data:
            return
 
        self.start_study(self.group_data['group_name'])
 
        if not self.flashcards:
            self.show_error_popup("No flashcards available for study.")
            return
 
        self.show_current_flashcard()
        self.study_button.text = 'Next Flashcard'
 
        self.study_button.unbind(on_release=self.study_flashcards)
        self.study_button.bind(on_release=self.show_next_flashcard)
 
    def show_current_flashcard(self):
        if 0 <= self.current_card_index < len(self.flashcards):
            card = self.flashcards[self.current_card_index]
            text = f"Question: {card['question']}\nAnswer: {card['answer']}"
            self.flashcard_list.clear_widgets()
            item = TwoLineListItem(text=text)
            self.flashcard_list.add_widget(item)
 
    def show_next_flashcard(self, instance):
        if 0 <= self.current_card_index < len(self.flashcards) - 1:
            self.current_card_index += 1
            self.show_current_flashcard()
        else:
            self.show_success_popup("Congratulations! You've studied all flashcards.")
            self.reset_study_session()
 
    def reset_study_session(self):
        self.current_card_index = 0
        self.study_button.text = 'Study'
        self.study_button.unbind(on_release=self.show_next_flashcard)
        self.study_button.bind(on_release=self.study_flashcards)
        self.flashcard_list.clear_widgets()
 
class StatsScreen(Screen):
    def on_left_action_items(self, instance):
        self.manager.current = 'menu'
 
class Answa(MDApp):
    def build(self):
        sm = ScreenManager()
        sm.add_widget(MenuScreen(name='menu'))
        sm.add_widget(CreateScreen(name='create'))
        sm.add_widget(StatsScreen(name='stats'))
        sm.add_widget(FlashcardsScreen(name='flashcards'))
        return sm
 
if __name__ == '__main__':
    Answa().run()