update

src/Form/Field.php

@@ -918,7 +918,7 @@ class Field implements Renderable
      *
      * @return mixed
      */
-    protected function getElementClassString()
+    public function getElementClassString()
     {
         $elementClass = $this->getElementClass();
 
@@ -940,7 +940,7 @@ class Field implements Renderable
      *
      * @return string|array
      */
-    protected function getElementClassSelector()
+    public function getElementClassSelector()
     {
         $elementClass = $this->getElementClass();