Error:RenderBox was not laid out: NEEDS-PAINT NEEDS-COMPOSITING-BITS-UPDATE

You cannot use Expanded in Column if it has a parent SingleChildScrollView because When you use column it tries to be in screen height and when use expanded inside, The column will allocate remaining space to the child of the expanded widget, Now if you use SingleChildScrollView It will try to expand(by direction, vertically in your case) as long as possible but as you’re using the Expanded which tries to take remaining space, So it goes infinite thus throws that error,

So Either remove SingleChildScrollView and use Column and expanded or remove the Expanded and Use SingleChildScrollView also make sure ShrinkWrap in ListView to true.

Column(
      children: [
        Text(
          'Hello, World!',
          style: Theme.of(context).textTheme.headline4,
        ),
        Container(
          height: 50,
          width: 200,
          color: Colors.amber,
          child: const Text('Random widget'),
        ),
        Expanded(
          child: ListView.separated(
            shrinkWrap: true,
            itemCount: 20,
            separatorBuilder: (_, __) => const Divider(),
            itemBuilder: (context, int index) {
              return ListTile(
                title: Text('Item at $index'),
              );
            },
          ),
        )
      ],
    );

Or to Scroll all the widgets You can do using SingleChildScrollView

Remove the Expanded widget here.

SingleChildScrollView(
      child: Column(
        children: [
          Text(
            'Hello, World!',
            style: Theme.of(context).textTheme.headline4,
          ),
          Container(
            height: 50,
            width: 200,
            color: Colors.amber,
            child: const Text('Random widget'),
          ),
          ListView.separated(
              shrinkWrap: true,
              itemCount: 20,
              separatorBuilder: (_, __) => const Divider(),
              itemBuilder: (context, int index) {
                return ListTile(
                  title: Text('Item at $index'),
                );
              },
            ),
        ],
      ),
    );