title: Variable annotated with [SerializeField] not showing up in inspector
body: <p>I have a problem with a variable which is not showing up in the inspector even though I have declared it a serialized field.</p>
  
  <p>Code:</p>
  
  <pre><code>using System;
  using UnityEngine;
  using UnityEngine.UIElements;
  
  public class SettingsPopup : MonoBehaviour
  {
      [SerializeField] private Slider speedSlider;
  
      private void Start()
      {
          speedSlider.value = PlayerPrefs.GetFloat("speed", 1);
      }
      ....
  }
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/GOIRi.png" rel="nofollow noreferrer">Unity inspector screenshot</a></p>
  
  <p>I am using Unity 2019.2.10f1 Personal.<br>
  edit: changing the type of speedSlider to anything outside the UnityEngine.UIElements namespace makes it appear in the inspector.</p>