title: Problems when setting up a show action in Sonata Admin. Error when I was trying customize the displayed label of a show field using the label option
body: <p>I am trying to set up a <code>showAction</code>. I want to customize the displayed label of a show field using "text" type for Categories in my code.
  Look at my code:</p>
  
  <pre><code>public function configureShowFields(ShowMapper $show)
  {
      $show-&gt;with("Day")
          -&gt;add("name", "text", array("label" =&gt; "Name"))
          -&gt;add("categories", "text", array("label" =&gt; "Categories"));
      $show-&gt;end();
  }
  </code></pre>
  
  <p>This is "categories" property with getters and setters:</p>
  
  <pre><code>    /**
   * @var Category[]
   *
   * @ORM\ManyToMany(targetEntity="App\Entity\Category\Category", cascade={"persist"})
   * @ORM\JoinTable(name="day_category",
   *     joinColumns={@ORM\JoinColumn(name="day_id", referencedColumnName="id")},
   *     inverseJoinColumns={@ORM\JoinColumn(name="category_id", referencedColumnName="id")})
   */
  private $categories;
  
  
  /**
   * @return Category[]
   */
  public function getCategories()
  {
      return $this-&gt;categories;
  }
  
  /**
   * @param Category[] $categories
   */
  public function setCategories(array $categories): void
  {
      $this-&gt;categories = $categories;
  }
  </code></pre>
  
  <p>I want to customize the categories label with text type, but if I do this I get the error:</p>
  
  <blockquote>
    <p>An exception has been thrown during the rendering of a template ("Warning: nl2br() expects parameter 1 to be string, object given").</p>
  </blockquote>
  
  <p>Here is a picture with what I want for "Categories":
  <a href="https://i.stack.imgur.com/QpRO9.png" rel="nofollow noreferrer">Here is what I want for categories</a></p>