public void stopLoop(){
        isRunning = false;
        // Stop the game-loop:
        try {
            game_loop_executor.shutdown();
            game_loop_executor.awaitTermination(10, TimeUnit.SECONDS);
        } catch (SecurityException e){
            e.printStackTrace();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        // Stop all scenes if they're running:
        for (Scene scene : scenes.values())
            if (scene.getSceneState() != Scene.State.PENDING)
                scene.onStopCall();
        // Hide the game-window:
        Viewport.setWindowVisibility(false);
    }