[Kivy] Blank Screen

[j.crater]

I didn't run your code, but I am concerned about naming your app and widgets GridLayout - same as the Kivy grid object.
Another thing is naming of Kivy resource file (.kv) and .py.
Try to rename GridLayoutApp(App) class to customwidgetApp. Example here:
https://kivy.org/doc/stable/examples/gen...v__py.html