[Python] Python doesn't recognize element from .kv file

In the .py file I want to refer to an element from the .kv file, but I get a keyerror

py:

class graApp(App):
def build(self):
Window.clearcolor = (1, 1, 1, 1)
layout = Builder.load_file('gra.kv')
grid = layout.ids['board']
#
# for i in range(121):
# grid.add_widget(Label(text=str(i)))


graApp().run()


kv:

BoxLayout:
[...]

GridLayout:
id: 'board'
cols: 11
[...]


Error:

grid = layout.ids['board']
~~~~~~~~~~^^^^^^^^^
KeyError: 'board'


Thanks in advance for your help

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