When you have a Flex widget, where the child with the maximum cross-axis size determines the cross-axis size of the rest.
If you try to use CrossAxisAlignment.stretch with a Flex whose parent is unbounded along the cross-axis, you will get an error.
To solve this, you should use Intrinsic* widgets. They do speculative pass on the Flex widget tree to determine the max size along the cross-axis and that becomes the constraint they pass to the Flex.