added test and fix

Closes #3334045

core/lib/Drupal/Core/Template/Attribute.php

@@ -94,6 +94,13 @@ public function offsetGet($name): mixed {
     if (isset($this->storage[$name])) {
       return $this->storage[$name];
     }
+    // The 'class' array key is expected to be itself an array, and therefore
+    // can be accessed using array append syntax before it has been initialized.
+    if ($name === 'class') {
+      // Initialize the class attribute as an empty array if not set.
+      $this->offsetSet('class', []);
+      return $this->storage['class'];
+    }
     return NULL;
   }
 

[quietone]

This should have a comment to explain that this is for using array syntax.

[Joachim Noreiko]

Done.