Passing data from view to Layout in Codeigniter 4 – PHP

I’m brand new to CodeIgniter, so apologies if I’m missing something obvious here.

I’m comfortable with sending data from a controller to a view file using return view('default/blog/index', $data);. My issue is accessing the same data in a layout file which is extended by the view file using <?= $this->extend('layouts/default'); ?>.

For example, if I insert <?= $data['content'] ?> in my view file, it displays as expected. If I insert the same code in the layout file that is extended by my view file, I get the "Trying to access array offset on value of type null" exception.

What am I missing that will allow me to access my data from within the layout file?

Thanks in advance.

Update:

So in my BlogController I’ve got

class BlogController extends BaseController
{
    public function index()
    {
        $model = new Blog();

        $data = $model->getBlog();

        return view('default/blog/index', ['data' => $data]);

    }


    public function item($slug = null){

        $model = new Blog();

        $data = $model->getBlog($slug);

        return view('default/blog/item', ['data' => $data]);

    }

}

And then in my item.php and index.php files, I have

<?= $this->extend('layouts/default', ['data' => $data]); ?>

My Blog Model’s getBlog() method:

public function getBlog($slug = false)
{
    if ($slug === false) {
        return $this->orderBy('bs_created_dt', 'desc')->findAll();
    }

    return $this->where(['bs_slug' => $slug])->first();
}

When I use the debug toolbar to inspect the data, it is showing as expected, and I can display it in the view files, but not in the layout file.