5d: Create a mock node

> TypeError: Return value of Drupal\my_module\Entity\Post::getTitle()
> must be of the type string, null returned

web/modules/custom/my_module/tests/src/Unit/Entity/PostTest.php

@@ -3,12 +3,15 @@
 namespace Drupal\Tests\my_module\Unit\Entity;
 
 use Drupal\my_module\Entity\Post;
+use Drupal\node\NodeInterface;
 use Drupal\Tests\UnitTestCase;
 
 class PostTest extends UnitTestCase {
 
   /** @test */
   public function it_returns_the_title() {
+    $node = $this->createMock(NodeInterface::class);
+
     $post = new Post($node);
 
     $this->assertSame('Test post', $post->getTitle());