How to setup correctly a custom classic theme?

Hi,

I’m creating a classic WordPress theme. I’d like to create a configuration class with a plugin_setup method that handles various configurations, such as importing CSS and JavaScript, creating custom post types and taxonomies, and other tasks.

I’ve created a simple setup that currently does nothing, but when I load a basic empty homepage.php page, the plugin_setup method is called twice. Is it possible to prevent the plugin_setup method from being called twice?

This is the code I’m using:

Code of functions.php:

if ( ! class_exists( 'DIS_ThemeManager' ) ) {
	require_once get_template_directory() . '/classes/theme-manager.php';
}
if ( class_exists( 'DIS_ThemeManager' ) ) {
	error_log("Running after_setup_theme");
	add_action(
		'after_setup_theme',
		function() {
			$theme_manager = DIS_ThemeManager::get_instance();
			$theme_manager->plugin_setup();
		}
	);
}

Code of theme-manager.php :

class DIS_ThemeManager {
	protected static $instance = null;


	private function __construct() {}

	public static function get_instance() {
		if ( is_null( self::$instance ) ) {
			self::$instance = new self();
		}
		return self::$instance;
	}

	public function plugin_setup() {
	}
}

Code of homepage.php:

<?php
	// get_header();
?>
<main id="main-container" class="main-container redbrown" role="main">
HELLO
</main>

With this setup, the add_action method and plugin_setup are called twice.

If I uncomment get_header() in homepage.php, they are called three times.

Why does this happen? Is it possible to prevent this behavior?

Thank you

Claudio