Call to a member function stat() on a non-object

<?php
class ProductExtension extends DataExtension{
    private static $many_many = array(
        'RelatedProducts' => 'Product'
    );

    private static $belongs_many_many = array(
        'IsRelatedTo' => 'Product'
    );

    public function updateCMSFields(FieldList $fields) {
        $gridField = GridField::create('RelatedProducts',
            'Related Products',
            $this->owner->RelatedProducts(),
            GridFieldConfig_RelationEditor::create()
        );

        $fields->addFieldToTab('Root.RelatedProducts', $gridField);
    }
}