⏫ Forwardport of #11349 to 2.3-develop branch

Author: p-bystritsky

lib/internal/Magento/Framework/Data/Form/Element/Multiselect.php

@@ -57,7 +57,9 @@ public function getElementHtml()
         $this->addClass('select multiselect admin__control-multiselect');
         $html = '';
         if ($this->getCanBeEmpty()) {
-            $html .= '<input type="hidden" name="' . parent::getName() . '" value="" />';
+            $html .= '
+                <input type="hidden" id="' . $this->getHtmlId() . '_hidden" name="' . parent::getName() . '" value="" />
+                ';
         }
         $html .= '<select id="' . $this->getHtmlId() . '" name="' . $this->getName() . '" ' . $this->serialize(
             $this->getHtmlAttributes()

lib/internal/Magento/Framework/Data/Test/Unit/Form/Element/MultiselectTest.php

@@ -26,10 +26,16 @@ protected function setUp()
      */
     public function testHiddenFieldPresentInMultiSelect()
     {
-        $this->_model->setDisabled(true);
+        $fieldName = 'fieldName';
+        $fieldId = 'fieldId';
         $this->_model->setCanBeEmpty(true);
+        $this->_model->setName($fieldName);
+        $this->_model->setId($fieldId);
         $elementHtml = $this->_model->getElementHtml();
-        $this->assertContains('<input type="hidden"', $elementHtml);
+        $this->assertContains(
+            '<input type="hidden" id="' . $fieldId . '_hidden" name="' . $fieldName . '"',
+            $elementHtml
+        );
     }
 
     /**