Yii2 creating a get method for a mysql table column doesn’t work

I’m creating a class that gets values from my database.
My database looks like this:

  id  |  value  |  quantity  |  type  |
---------------------------------------
  1   |  134    |  12        |  1     |
---------------------------------------
  2   |  21     |  2         |  1     |
---------------------------------------
  3   |  211    |  53        |  1     |

Right now my class looks like this:

?php

namespace commonclasses;

use commonmodelsdbstockType;
use yiidbActiveRecord;

class Stock extends ActiveRecord
{
    /**
     * @var stockType
     */
    public $StockData;

    public function run()
    {
        $this->StockData::find()
            ->select([
                'SUM(quantity) as stockQuantity',
                'SUM(value) as stockValue'
            ])
            ->where(['type' => 1])
            ->asArray()
            ->one();
    }


    public function getQuantity()
    {
        return $this->StockData::findOne(['column' => 'stockQuantity']);
    }

And in my view I do this:

<?php

use commonclassesStock;

$stock = new Stock()

?>

<div class='row'>
      <?= $stock->getQuantity() ?>
</div>

I would love to see the quantity in my view but I keep getting the following error:
Class name must be a valid object or a string

It points to the get method return, but I have no idea how to fix it.