Black screen on kivy how to fix

Question

when I try to run the code it just displays a blank screen I tried to find answers everywhere but nothing works This is my main.py file:

import kivy
from kivy.app import App
from kivy.uix.label import Label
from kivy.uix.gridlayout import GridLayout
from kivy.uix.textinput import TextInput
from kivy.uix.button import Button
from kivy.uix.widget import Widget
from kivy.properties import ObjectProperty
from kivy.graphics import *
from kivy.lang import *
from kivy.uix.screenmanager import *
class MyWidgets(Widget):
    pass
kv = Builder.load_file('math.kv')
class Calculator(App):
    def build(self):
    return kv
if __name__ == '__main__':
    Calculator().run()

And this is my math.kv file:

<MyWidgets>
num: num
GridLayout:
    cols: 1
    TextInput:
        id: num
        multiline: False
    GridLayout:
        cols: 3
        Button:
            text: '1'
            font_size: 25
        Button:
            text: '2'
            font_size: 25
        Button:
            text: '3'
            font_size: 25

Answer 1

Looks like you have an indent error in build function. Still, that should have shown some error instead of showing a blank screen. Then in your .kv file, I don't know what that num is. After removing num from .kv and correcting indentation error I run the file and it works fine. Here's the code I used:

.py

import kivy
from kivy.app import App
from kivy.uix.label import Label
from kivy.uix.gridlayout import GridLayout
from kivy.uix.textinput import TextInput
from kivy.uix.button import Button
from kivy.uix.widget import Widget
from kivy.properties import ObjectProperty
from kivy.graphics import *
from kivy.lang import *
from kivy.uix.screenmanager import *
class MyWidgets(Widget):
    pass
class Calculator(App):
    def build(self):
        kv = Builder.load_file('math.kv')
        return kv
if __name__ == '__main__':
    Calculator().run()

.kv

<MyWidgets>

GridLayout:
    cols: 1
    TextInput:
        id: num
        multiline: False
    GridLayout:
        cols: 3
        Button:
            text: '1'
            font_size: 25
        Button:
            text: '2'
            font_size: 25
        Button:
            text: '3'
            font_size: 25