[Kivy] Blank Screen

[Exsul]

No error message, but I get a plain black window when I run this code:

practice1.py

import kivy
kivy.require("1.10.1")

from kivy.app import App
from kivy.uix.gridlayout import GridLayout

class GridLayoutApp(App):

    def build(self):
        return GridLayout()

glApp = GridLayoutApp()
glApp.run()

customwidget.kv

<GridLayout>:
    cols: 2
    rows: 2
    spacing: 10
    padding: 10

    Button:
        text: "1st"
        size_hint_x: None
        width: 200
    Button:
        text: "2nd"
    Button:
        text: "3rd"
        size_hint_x: None
        width: 200
    Button:
        text: "4th"