[Python] how to center Spinner in kivy

I started learning kivy and stopped at the stage with .kv files. The block with Spining is stuck to the corner and I can't put it in the center

//python file

class MyApp(App):
def build(self):
Builder.load_file('widgets.kv')
return countryLists()

class countryLists(Widget):
def countryList(self):
return pd.read_csv('./data.csv')['Country'].tolist()

if __name__ == '__main__':
MyApp().run()


// .kv file :

Spinner:
id: spinner1
text: 'select country'
values: root.countryList()

font_size: 24
bold: True
pos_hint: {"center_x": 0.5, "center_y": 0.5}
anchor: 'center'


I tryed use anchor: 'center', tryed use BoxLayout, change pos_hint, nothing to work

enter image description here

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