Unable to load customizer page in 5.5

Hello,
I had the same issue with frontech child theme.
Copy following code and paste into kyma theme function file :

if( is_admin() ){
	add_action( 'wp_default_scripts', 'wp_default_custom_scripts' );
	function wp_default_custom_scripts( $scripts ){
		$scripts->add( 'wp-color-picker', "/wp-admin/js/color-picker.js", array( 'iris' ), false, 1 );
		did_action( 'init' ) && $scripts->localize(
			'wp-color-picker',
			'wpColorPickerL10n',
			array(
				'clear'            => __( 'Clear' ),
				'clearAriaLabel'   => __( 'Clear color' ),
				'defaultString'    => __( 'Default' ),
				'defaultAriaLabel' => __( 'Select default color' ),
				'pick'             => __( 'Select Color' ),
				'defaultLabel'     => __( 'Color value' ),
			)
		);
	}
}

I had the same error and put your code in functions, but now ite even works this, get the error message;
Fatal error: Uncaught Error: Call to undefined function get_template_directory() in /web/u/lorema/wp-includes/functions.php:5 Stack trace: #0 /web/u/lorema/wp-settings.php(111): require() #1 /web/u/lorema/wp-config.php(90): require_once(‘/web/u/lorema/w…’) #2 /web/u/lorema/wp-load.php(37): require_once(‘/web/u/lorema/w…’) #3 /web/u/lorema/wp-blog-header.php(13): require_once(‘/web/u/lorema/w…’) #4 /web/u/lorema/index.php(17): require(‘/web/u/lorema/w…’) #5 {main} thrown in /web/u/lorema/wp-includes/functions.php on line 5 Fatal error: Uncaught Error: Call to undefined function wp_die() in /web/u/lorema/wp-includes/class-wp-fatal-error-handler.php:233 Stack trace: #0 /web/u/lorema/wp-includes/class-wp-fatal-error-handler.php(152): WP_Fatal_Error_Handler->display_default_error_template(Array, false) #1 /web/u/lorema/wp-includes/class-wp-fatal-error-handler.php(57): WP_Fatal_Error_Handler->display_error_template(Array, false) #2 [internal function]: WP_Fatal_Error_Handler->handle() #3 {main} thrown in /web/u/lorema/wp-includes/class-wp-fatal-error-handler.php on line 233