/**
  * Verify displayed product sku on product page(front-end) equals passed from fixture
  *
  * @return string|null
  */
 protected function verifySku()
 {
     $fixtureProductSku = $this->product->getSku();
     $formProductSku = $this->productView->getProductSku();
     if ($fixtureProductSku === null || $fixtureProductSku == $formProductSku) {
         return null;
     }
     return "Displayed product sku on product page(front-end) not equals passed from fixture. " . "Actual: {$formProductSku}, expected: {$fixtureProductSku}.";
 }