Add missing property

Closes #3461280

src/ComputedEnum.php

@@ -9,6 +9,13 @@ use Drupal\Core\TypedData\TypedData;
  */
 class ComputedEnum extends TypedData {
 
+  /**
+   * The data value.
+   *
+   * @var mixed
+   */
+  protected $value;
+
   /**
    * Computed enum.
    *