The fix for https://bugs.openjdk.java.net/browse/JDK-8241840 broke the repaint-from-native on Android.
On Android, the native layer may request a repaint (e.g. based on lifecycle events), which is handled by the MonocleWindowManager :
    static void repaintFromNative () {
        Platform.runLater(new Runnable () {

            @Override
            public void run() {
                Screen.notifySettingsChanged();
                instance.getFocusedWindow().setFullScreen(true);
                instance.repaintAll();
                Toolkit.getToolkit().requestNextPulse();
            }
        });
    }

This is now failing since instance.getFocusedWindow() returns null for invisible windows (which is the case when the app is initially rendered)
See https://github.com/gluonhq/substrate/issues/810