Yii2: Why can't I get the computed property of an ActiveRecord model?

Question

Yii

Hello.
Communication announced:

/** @var app\models\Goods $this*/
public function getProducer()
    {
        return $this->hasOne(GoodProducers::className(), ['id' => 'fk_producers']);
    }

In View I use the following code to display the full name of the product:

/** @var app\models\Goods $model */
<?= $model->producer->name . ' ' . $model->name; ?>

The name of the product along with the manufacturer are displayed normally.
If I try to declare a new property of the Goods $fullName model, then the name of the manufacturer is not displayed.

/** @var app\models\Goods $this*/
public $fullName;
public function getFullName()
{
     return $this->producer->name . ' ' . $this->name;
}

Why here when debugging $this->producer->name PhpStorm writes
that "Cannot evaluate expression" , although there is a producer in $this->$_related?
The documentation describes that it is possible to convert data
https://github.com/yiisoft/yii2/blob/master/docs/g...
Why doesn't it work with related data?