title: Fragment cannot be converted to CustomArFragment
body: <p>I have recently ran into some troubles and would like to ask whether anyone has faced the same issue here. Recently, I rebuild my AR project in 3.6.3 and found &quot;error: incompatible types: Fragment cannot be converted to CustomArFragment&quot;. Afterwards, I shifted back to 3.5.3, and the error does not go away.</p>
  <pre><code>arFragment = (CustomArFragment) getSupportFragmentManager().findFragmentById(R.id.sceneform_fragment);
  assert arFragment != null;
  arFragment.getPlaneDiscoveryController().hide(); // delete plane detection &amp; hands gesture, needs to be placed in the onCreate (when app starts)
  </code></pre>
  <p>Prior to rebuild, the Fragment class has been compatible with CustomArFragment, and my app has been working fine.</p>
  <pre><code>import com.google.ar.core.Config;
  import com.google.ar.core.Session;
  import com.google.ar.sceneform.ux.ArFragment;
  
  public class CustomArFragment extends ArFragment {
  
      @Override
      protected Config getSessionConfiguration(Session session) {
          getPlaneDiscoveryController().setInstructionView(null);
  
          Config config = new Config(session); // Config handles Settings for ARCore, session controls lifecycle of Augmented Reality of ARCore
          config.setUpdateMode(Config.UpdateMode.LATEST_CAMERA_IMAGE); // The Settings are set to allow augmented images to work
          config.setFocusMode(Config.FocusMode.AUTO); // Camera Focus
          config.setPlaneFindingMode(Config.PlaneFindingMode.HORIZONTAL);
          config.setLightEstimationMode(Config.LightEstimationMode.ENVIRONMENTAL_HDR);
          session.configure(config);
          this.getArSceneView().setupSession(session); // attach session to ArSceneView
  
          return config;
      }
  
  }
  </code></pre>
  <p>With reference to the documentation on sceneform 1.15.0, my code has not been adapted and just rebuilt, but the debug message keeps popping up and preventing the running of the codes.</p>