[Flutter] RenderBox was not laid out when using Positioned in Stack

I want to position Container with two buttons wrapped in Row in my Stack widget.

This is what I want to achieve. This text should be scrolled if it's long enough, that's why I have SingleChildScrollView. Last thing I need to do is to position this two buttons on bottom



However, when I do that it's not working and I am getting this error: RenderBox was not laid out: RenderDecoratedBox#04ae9 relayoutBoundary=up8

How to fix this?

Stack(
children: [
SizedBox(
height: size.width,
child: (...)
),
Container(
margin: EdgeInsets.only(
top: size.width - 16,
bottom: 64,
),
decoration: BoxDecoration(
borderRadius: BorderRadius.only(
topLeft: Radius.circular(16),
topRight: Radius.circular(16),
),
color: Colors.white
),
child: MainPadding(
child: Stack(
children: [
Column(
crossAxisAlignment: CrossAxisAlignment.start,
children: [
Text('Nazwa produktu'),
Row(
mainAxisAlignment: MainAxisAlignment.spaceBetween,
children: [
NumberPicker(),
Text('250 zł')
],
),
const SizedBox(height: 16),
Text('Opis'),
const SizedBox(height: 8),
Expanded(
child: SingleChildScrollView(
child: Text('...'),
),
),
],
),
Positioned(
bottom: 32,
child: Container(
decoration: BoxDecoration(
color: Colors.white,
border: Border.all(
color: Colors.black
)
),
child: Row(
mainAxisAlignment: MainAxisAlignment.spaceBetween,
children: [
Expanded(
child: ElevatedButton(
onPressed: someMethod,
child: Text('kup i zapłać'),
),
),
const SizedBox(width: 8),
Expanded(
child: ElevatedButton(
onPressed: someMethod2,
child: Text('Do koszyka'),
),
)
],
),
),
),
],
),
),
)
],
),

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