Issue #3517901: Fix the remaining phpstan issue

Closes #3517901

tests/src/Unit/ExtraFieldFormManagerTest.php

@@ -2,7 +2,9 @@
 
 namespace Drupal\Tests\extra_field\Unit;
 
+use Drupal\extra_field\Plugin\ExtraFieldFormManager;
 use Drupal\Tests\UnitTestCase;
+use PHPUnit\Framework\MockObject\MockObject;
 
 /**
  * @coversDefaultClass \Drupal\extra_field\Plugin\ExtraFieldFormManager
@@ -13,10 +15,8 @@ class ExtraFieldFormManagerTest extends UnitTestCase {
 
   /**
    * The plugin manager under test.
-   *
-   * @var \Drupal\extra_field\Plugin\ExtraFieldFormManagerInterface|\PHPUnit\Framework\MockObject\MockObject
    */
-  protected $formManager;
+  protected ExtraFieldFormManager&MockObject $formManager;
 
   /**
    * {@inheritdoc}