VrVideoView remove UI

I'm trying to display a 3D stereo video in a googlevr app in a clean fashion, without showing the UI. I know about usability guidelines, but the device running the app will be always kept inside a viewer in a sort of demo, so no touch interaction expected.

I'm using a VrVideoView. So I already got rid of fullscreen button, info button, stereo mode button, google cardboard tutorial screen named "transition view" and touch tracking to move the view.

videoWidgetView.setFullscreenButtonEnabled(false);
videoWidgetView.setInfoButtonEnabled(false);
videoWidgetView.setStereoModeButtonEnabled(false);
videoWidgetView.setTransitionViewEnabled(false);
videoWidgetView.setTouchTrackingEnabled(false);

I also enabled fullscreen stereo by default.

videoWidgetView.setDisplayMode(VrWidgetView.DisplayMode.FULLSCREEN_STEREO);

But I can't remove the close button "x" and the option button.

I think that the "x" is fullscreenBackButton of VrWidgetView, parent of VrVideoView. Which hasn't methods to control its visibility.

Is there a way to remove those two buttons?
Maybe subclassing and rewriting part of the widget code?
Maybe just a little hack putting a black overlay above those corners?

---

I've also tried as suggested

findViewById(R.id.ui_back_button).setVisibility(GONE);

or even

findViewById(com.google.vr.widgets.common.R.id.ui_back_button).setVisibility(GONE);

without success, they give:
NullPointerException: Attempt to invoke virtual method 'void android.view.View.setVisibility(int)' on a null object reference

For me this solution works. It uses field functionality to get vrUiLayer which is a private member of VrWidgetView which is the parent of VrVideoView.

Field f;
try {
    f = this.videoWidgetView.getClass().getSuperclass().getDeclaredField("vrUiLayer");
    f.setAccessible(true);
    UiLayer vrLayer = (UiLayer) f.get(this.videoWidgetView);

    // - here you can directly access to the UiLayer class - //


    // to hide the up-right settings button
    vrLayer.setSettingsButtonEnabled(false);

    //setting listener to null hides the x button
    vrLayer.setBackButtonListener(null);

    // these visibility options are frequently updated when you play videos,
    // so you have to call them often


    // OR


    vrLayer.setEnabled(false); // <- just hide the UI layer


} catch (NoSuchFieldException e) {
    e.printStackTrace();
} catch (IllegalAccessException e) {
    e.printStackTrace();
}

Don't forget to import the class

import com.google.vr.cardboard.UiLayer

Beware that using vrLayer.setEnabled(false) hides also the center line, leaving a complete clean vr experience.