Issue #3367502 by Grimreaper: Fatal error when using the Diff module

src/Utility/Attributes.php

@@ -63,6 +63,9 @@ class Attributes extends ArrayObject {
    */
   public function &getClasses(): array {
     $classes = &$this->offsetGet('class', []);
+    if (\is_string($classes)) {
+      $classes = \explode(' ', $classes);
+    }
     $classes = \array_unique($classes);
     return $classes;
   }