[Python] Why is container within intro_card variable not getting displayed

I am developing a flet app. I have a main container and sub container within intro_card variable

Why is container within intro_card variable not getting displayed?

from flet import *

def main (page: Page):
page.window.width = 411
page.window.height = 891
page.window.resizable = True, # window is resizable
page.bgcolor = 'blue'


intro_card = Column(
controls = [
Container(
width = 100,
height = 200,
bgcolor = 'green'
)
]
)


main_content = Container(
width = 350,
height = 500,
bgcolor = '#d6ecf2',
alignment = 'center',
content = intro_card
)

page.add(main_content)

page.update()

app (target = main)

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