Issue 3276930 "TypeError: array_unique(): Argument #1 ($array) must be of type...

Issue 3276930 "TypeError: array_unique(): Argument #1 ($array) must be of type array, string given" fixed

Closes #3276930

src/Utility/Attributes.php

@@ -63,6 +63,11 @@ class Attributes extends ArrayObject {
    */
   public function &getClasses() {
     $classes = &$this->offsetGet('class', []);
+    if(is_array($classes)) {
+      $classes = array_unique($classes);
+    } else {
+      $classes = array($classes);
+	  }
     $classes = array_unique($classes);
     return $classes;
   }